diff --git a/backend/app/scripts/migrate_md.py b/backend/app/scripts/migrate_md.py
index 086086e..316808f 100644
--- a/backend/app/scripts/migrate_md.py
+++ b/backend/app/scripts/migrate_md.py
@@ -23,8 +23,6 @@
 from app.database import SessionLocal, engine
 from app.models import Base, Card
 
-Base.metadata.create_all(bind=engine)
-
 
 def parse_cards(text: str) -> list[dict]:
     cards = []
@@ -111,6 +109,92 @@ def flush_card():
     return cards
 
 
+def find_unclosed_code(text: str) -> list[str]:
+    """Ищет незакрытые блоки кода и вопросы, потерянные из-за них.
+    Это эвристика: ограда с языком внутри блока считается началом нового блока.
+    """
+    warnings: list[str] = []
+    category = "General"
+    question = None
+    in_code = False
+    code_owner: tuple[str | None, str] = (None, category)
+    lost: list[str] = []
+
+    def report_unclosed() -> None:
+        q, c = code_owner
+        warnings.append(
+            f'Незакрытый блок кода в вопросе "{q or "?"}" (категория "{c}"): '
+            "проверьте карточку"
+        )
+        warnings.extend(
+            f'Заголовок "{title}" внутри незакрытого блока: карточка не будет создана'
+            for title in lost
+        )
+
+    for line in text.splitlines():
+        stripped = line.strip()
+        if stripped.startswith("```"):
+            if not in_code:
+                in_code = True
+                code_owner = (question, category)
+            elif len(stripped) > 3:
+                report_unclosed()
+                code_owner = (question, category)
+            else:
+                in_code = False
+            lost = []
+            continue
+
+        header = re.match(r"^(#{1,6})\s+(.+)$", line)
+        if in_code:
+            if header and len(header.group(1)) >= 3:
+                lost.append(header.group(2).strip())
+            continue
+
+        if header:
+            title = header.group(2).strip()
+            if len(header.group(1)) <= 2:
+                category = title
+                question = None
+            else:
+                question = title
+
+    if in_code:
+        report_unclosed()
+    return warnings
+
+
+def import_cards(db, cards_data: list[dict]) -> tuple[int, int]:
+    """Добавляет карточки в БД, пропуская дубли по паре (вопрос, категория).
+    Возвращает (добавлено, пропущено как дубли).
+    Карточки с пустым вопросом или ответом не считаются ни добавленными, ни дублями.
+    """
+    seen = set(db.query(Card.question, Card.category).all())
+    inserted = 0
+    skipped = 0
+    for data in cards_data:
+        if not data["question"] or not data["answer"]:
+            continue
+        key = (data["question"], data["category"])
+        if key in seen:
+            skipped += 1
+            continue
+        seen.add(key)
+        db.add(
+            Card(
+                question=data["question"],
+                answer=data["answer"],
+                code_example=data["code_example"],
+                category=data["category"],
+                tags=json.dumps(data["tags"], ensure_ascii=False),
+                difficulty=data["difficulty"],
+            )
+        )
+        inserted += 1
+    db.commit()
+    return inserted, skipped
+
+
 def main():
     parser = argparse.ArgumentParser()
     parser.add_argument("--file", required=True, help="Path to questions.md")
@@ -122,9 +206,13 @@ def main():
     with open(args.file, encoding="utf-8") as f:
         text = f.read()
 
+    for warning in find_unclosed_code(text):
+        print(f"ВНИМАНИЕ: {warning}")
+
     cards_data = parse_cards(text)
     print(f"Найдено карточек: {len(cards_data)}")
 
+    Base.metadata.create_all(bind=engine)
     db = SessionLocal()
     try:
         if args.clear:
@@ -132,23 +220,8 @@ def main():
             db.commit()
             print("Старые карточки удалены")
 
-        inserted = 0
-        for data in cards_data:
-            if not data["question"] or not data["answer"]:
-                continue
-            card = Card(
-                question=data["question"],
-                answer=data["answer"],
-                code_example=data["code_example"],
-                category=data["category"],
-                tags=json.dumps(data["tags"], ensure_ascii=False),
-                difficulty=data["difficulty"],
-            )
-            db.add(card)
-            inserted += 1
-
-        db.commit()
-        print(f"Импортировано: {inserted} карточек")
+        inserted, skipped = import_cards(db, cards_data)
+        print(f"Импортировано: {inserted} карточек, пропущено дублей: {skipped}")
     finally:
         db.close()
 
diff --git a/backend/tests/test_cards.py b/backend/tests/test_cards.py
index 8c2f042..c4c03bc 100644
--- a/backend/tests/test_cards.py
+++ b/backend/tests/test_cards.py
@@ -140,6 +140,15 @@ def test_tags_filter_treats_underscore_literally(self, client, db):
         assert data["total"] == 1
         assert data["items"][0]["question"] == "A"
 
+    def test_per_page_zero_rejected(self, client, db):
+        assert client.get("/api/cards?per_page=0").status_code == 422
+
+    def test_per_page_above_limit_rejected(self, client, db):
+        assert client.get("/api/cards?per_page=501").status_code == 422
+
+    def test_per_page_at_limit_accepted(self, client, db):
+        assert client.get("/api/cards?per_page=500").status_code == 200
+
 
 class TestGetCardById:
     def test_returns_card(self, client, db):
@@ -159,6 +168,9 @@ def test_correct_id_returned(self, client, db):
         data = client.get(f"/api/cards/{card.id}").json()
         assert data["id"] == card.id
 
+    def test_unknown_id_returns_404(self, client, db):
+        assert client.get("/api/cards/999").status_code == 404
+
 
 class TestGetCategories:
     def test_returns_categories(self, client, db):
diff --git a/backend/tests/test_migrate_md.py b/backend/tests/test_migrate_md.py
new file mode 100644
index 0000000..f5488c0
--- /dev/null
+++ b/backend/tests/test_migrate_md.py
@@ -0,0 +1,179 @@
+import importlib
+from unittest.mock import patch
+
+from app.models import Card
+from app.scripts import migrate_md
+from app.scripts.migrate_md import parse_cards
+from tests.conftest import make_card
+
+
+def md(*lines: str) -> str:
+    return "\n".join(lines)
+
+
+def card_data(question: str, category: str = "Python", answer: str = "Ответ.") -> dict:
+    return {
+        "question": question,
+        "answer": answer,
+        "code_example": None,
+        "category": category,
+        "tags": [],
+        "difficulty": "normal",
+    }
+
+
+class TestParseCards:
+    def test_single_card(self):
+        cards = parse_cards(
+            md("# Python", "#### Что такое list?", "Изменяемая последовательность.")
+        )
+        assert cards == [
+            {
+                "question": "Что такое list?",
+                "answer": "Изменяемая последовательность.",
+                "code_example": None,
+                "category": "Python",
+                "tags": [],
+                "difficulty": "normal",
+            }
+        ]
+
+    def test_default_category_when_no_header(self):
+        cards = parse_cards(md("### Вопрос?", "Ответ."))
+        assert cards[0]["category"] == "General"
+
+    def test_h2_changes_category(self):
+        cards = parse_cards(
+            md("# Python", "## Функции", "### Что такое lambda?", "Ответ.")
+        )
+        assert cards[0]["category"] == "Функции"
+
+    def test_several_cards_in_order(self):
+        cards = parse_cards(
+            md("# A", "### Q1", "a1", "### Q2", "a2", "# B", "### Q3", "a3")
+        )
+        assert [(c["question"], c["category"]) for c in cards] == [
+            ("Q1", "A"),
+            ("Q2", "A"),
+            ("Q3", "B"),
+        ]
+
+    def test_text_before_first_question_ignored(self):
+        cards = parse_cards(md("вступление", "# A", "### Q", "ответ"))
+        assert len(cards) == 1
+        assert cards[0]["answer"] == "ответ"
+
+    def test_single_code_block(self):
+        cards = parse_cards(md("### Q", "текст", "```python", "print(1)", "```"))
+        assert cards[0]["answer"] == "текст"
+        assert cards[0]["code_example"] == "print(1)"
+
+    def test_several_code_blocks_numbered(self):
+        cards = parse_cards(
+            md(
+                "### Q",
+                "текст",
+                "```python",
+                "print(1)",
+                "```",
+                "```python",
+                "print(2)",
+                "```",
+            )
+        )
+        assert cards[0]["code_example"] == (
+            "# Пример 1\n\nprint(1)\n\n# Пример 2\n\nprint(2)"
+        )
+
+    def test_header_inside_code_block_is_not_a_question(self):
+        cards = parse_cards(md("### Q", "текст", "```python", "# комментарий", "```"))
+        assert len(cards) == 1
+        assert cards[0]["code_example"] == "# комментарий"
+
+
+class TestImportCards:
+    def test_inserts_new_cards(self, db):
+        result = migrate_md.import_cards(db, [card_data("Q1"), card_data("Q2")])
+        assert result == (2, 0)
+        assert db.query(Card).count() == 2
+
+    def test_second_run_skips_duplicates(self, db):
+        data = [card_data("Q1"), card_data("Q2")]
+        migrate_md.import_cards(db, data)
+        assert migrate_md.import_cards(db, data) == (0, 2)
+        assert db.query(Card).count() == 2
+
+    def test_existing_card_in_db_skipped(self, db):
+        make_card(db, question="Q", category="Python")
+        assert migrate_md.import_cards(db, [card_data("Q")]) == (0, 1)
+        assert db.query(Card).count() == 1
+
+    def test_same_question_in_other_category_is_not_duplicate(self, db):
+        data = [card_data("Q", "Python"), card_data("Q", "Django")]
+        assert migrate_md.import_cards(db, data) == (2, 0)
+
+    def test_duplicates_inside_file_skipped(self, db):
+        assert migrate_md.import_cards(db, [card_data("Q"), card_data("Q")]) == (1, 1)
+        assert db.query(Card).count() == 1
+
+    def test_empty_question_or_answer_not_imported(self, db):
+        data = [card_data(""), card_data("Q", answer="")]
+        assert migrate_md.import_cards(db, data) == (0, 0)
+        assert db.query(Card).count() == 0
+
+
+class TestFindUnclosedCode:
+    def test_closed_blocks_give_no_warnings(self):
+        text = md(
+            "# Python",
+            "### Q",
+            "```python",
+            "print(1)",
+            "```",
+            "```python",
+            "print(2)",
+            "```",
+        )
+        assert migrate_md.find_unclosed_code(text) == []
+
+    def test_header_inside_closed_block_is_not_a_problem(self):
+        text = md("### Q", "```python", "# комментарий", "```")
+        assert migrate_md.find_unclosed_code(text) == []
+
+    def test_unclosed_at_end_of_file(self):
+        text = md("# Python", "### Что такое lambda?", "текст", "```python", "x = 1")
+        warnings = migrate_md.find_unclosed_code(text)
+        assert len(warnings) == 1
+        assert "Что такое lambda?" in warnings[0]
+        assert "Python" in warnings[0]
+
+    def test_unclosed_in_middle_reports_question_and_lost_header(self):
+        text = md(
+            "# Python",
+            "### Q1",
+            "```python",
+            "x = 1",
+            "### Q2",
+            "ответ",
+            "```python",
+            "y = 2",
+            "```",
+        )
+        warnings = migrate_md.find_unclosed_code(text)
+        assert len(warnings) == 2
+        assert "Q1" in warnings[0]
+        assert "Q2" in warnings[1]
+
+    def test_unclosed_at_end_reports_lost_header_too(self):
+        text = md("### Q1", "```python", "x = 1", "### Q2", "текст")
+        warnings = migrate_md.find_unclosed_code(text)
+        assert len(warnings) == 2
+        assert "Q1" in warnings[0]
+        assert "Q2" in warnings[1]
+
+
+class TestImportHasNoSideEffects:
+    def test_import_does_not_create_tables(self):
+        with patch("app.models.Base.metadata.create_all") as create_all:
+            importlib.reload(migrate_md)
+        create_all.assert_not_called()
diff --git a/bot/cards.py b/bot/cards.py
index db90b3e..fe4ed42 100644
--- a/bot/cards.py
+++ b/bot/cards.py
@@ -1,8 +1,8 @@
 import json
 import logging
-import random
 from html import escape
 
+from sqlalchemy import func
 from sqlalchemy.orm import Session
 
 from config import settings
@@ -47,10 +47,7 @@ def get_next_cards(db: Session, count: int, last_id: int = 0) -> list[Card]:
 
 
 def get_random_card(db: Session) -> Card | None:
-    cards = db.query(Card).all()
-    if not cards:
-        return None
-    return random.choice(cards)
+    return db.query(Card).order_by(func.random()).first()
 
 
 def format_card(card: Card) -> list[str]:
diff --git a/bot/database.py b/bot/database.py
index b75641d..871fe2a 100644
--- a/bot/database.py
+++ b/bot/database.py
@@ -9,11 +9,3 @@
 )
 SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)
 Base = declarative_base()
-
-
-def get_db():
-    db = SessionLocal()
-    try:
-        yield db
-    finally:
-        db.close()
diff --git a/bot/pytest.ini b/bot/pytest.ini
new file mode 100644
index 0000000..c7b23ec
--- /dev/null
+++ b/bot/pytest.ini
@@ -0,0 +1,3 @@
+[pytest]
+pythonpath = .
+testpaths = tests
diff --git a/bot/tests/test_cards.py b/bot/tests/test_cards.py
index 67775f2..a01a840 100644
--- a/bot/tests/test_cards.py
+++ b/bot/tests/test_cards.py
@@ -1,6 +1,4 @@
 import json
-import os
-import sys
 from html import unescape
 from unittest.mock import MagicMock
 
@@ -13,11 +11,7 @@
     format_card,
     format_card_discord,
     get_next_cards,
-    get_random_card,
 )
-
-sys.path.insert(0, os.path.join(os.path.dirname(__file__), ".."))
-
 from config import settings
 from models import Card
 
@@ -187,21 +181,6 @@ def test_empty_db_returns_empty(self):
         assert result == []
 
 
-class TestGetRandomCard:
-    def test_returns_card(self):
-        db = MagicMock()
-        cards = [make_card(id=1), make_card(id=2)]
-        db.query().all.return_value = cards
-        result = get_random_card(db)
-        assert result in cards
-
-    def test_returns_none_for_empty_db(self):
-        db = MagicMock()
-        db.query().all.return_value = []
-        result = get_random_card(db)
-        assert result is None
-
-
 class TestFormatCardDiscord:
     def test_long_code_parts_within_discord_limit(self):
         """Каждое сообщение вместе с обёрткой ```python не длиннее лимита Discord."""
diff --git a/bot/tests/test_cards_db.py b/bot/tests/test_cards_db.py
index 6bcdce6..12b033d 100644
--- a/bot/tests/test_cards_db.py
+++ b/bot/tests/test_cards_db.py
@@ -1,14 +1,9 @@
-import os
-import sys
-
 import pytest
-from sqlalchemy import create_engine
+from sqlalchemy import create_engine, event
 from sqlalchemy.orm import sessionmaker
 from sqlalchemy.pool import StaticPool
 
-sys.path.insert(0, os.path.join(os.path.dirname(__file__), ".."))
-
-from cards import get_next_cards
+from cards import get_next_cards, get_random_card
 from models import Card
 
 
@@ -61,3 +56,26 @@ def test_last_id_beyond_max_starts_from_beginning(self, db):
 
     def test_empty_db_returns_empty(self, db):
         assert get_next_cards(db, 3, last_id=0) == []
+
+
+class TestGetRandomCardRealDb:
+    def test_returns_one_of_the_cards(self, db):
+        add_cards(db, 3)
+        assert get_random_card(db).id in {1, 2, 3}
+
+    def test_empty_db_returns_none(self, db):
+        assert get_random_card(db) is None
+
+    def test_selects_single_row_in_sql(self, db):
+        add_cards(db, 5)
+        statements: list[str] = []
+        event.listen(
+            db.get_bind(),
+            "before_cursor_execute",
+            lambda conn, cursor, statement, params, context, many: statements.append(
+                statement
+            ),
+        )
+        get_random_card(db)
+        assert len(statements) == 1
+        assert "LIMIT" in statements[0].upper()
diff --git a/bot/tests/test_config.py b/bot/tests/test_config.py
index b96a024..a390d8d 100644
--- a/bot/tests/test_config.py
+++ b/bot/tests/test_config.py
@@ -1,10 +1,5 @@
-import os
-import sys
-
 import pytest
 
-sys.path.insert(0, os.path.join(os.path.dirname(__file__), ".."))
-
 import config
 
 
diff --git a/bot/tests/test_discord_sender.py b/bot/tests/test_discord_sender.py
index b519aef..c8432e2 100644
--- a/bot/tests/test_discord_sender.py
+++ b/bot/tests/test_discord_sender.py
@@ -1,12 +1,8 @@
 import asyncio
 import logging
-import os
-import sys
 
 import pytest
 
-sys.path.insert(0, os.path.join(os.path.dirname(__file__), ".."))
-
 from discord_sender import _MAX_ATTEMPTS, DiscordSender
 
 
diff --git a/bot/tests/test_main.py b/bot/tests/test_main.py
index a9f3865..4aa7a1c 100644
--- a/bot/tests/test_main.py
+++ b/bot/tests/test_main.py
@@ -1,13 +1,9 @@
 import asyncio
-import os
-import sys
 from types import SimpleNamespace
 from unittest.mock import AsyncMock, MagicMock, patch
 
 import pytest
 
-sys.path.insert(0, os.path.join(os.path.dirname(__file__), ".."))
-
 
 @pytest.fixture(scope="module")
 def main_module():
diff --git a/bot/tests/test_state.py b/bot/tests/test_state.py
index bff71be..fa8de60 100644
--- a/bot/tests/test_state.py
+++ b/bot/tests/test_state.py
@@ -1,10 +1,7 @@
 import os
-import sys
 
 import pytest
 
-sys.path.insert(0, os.path.join(os.path.dirname(__file__), ".."))
-
 import state