Fix: Paginator accepts per_page <= 0 and crashes later instead of failing at construction

[pullapprove5]

Reproduced the finding directly against plain/plain/paginator.py: Paginator([1,2,3], 0) raised ZeroDivisionError from num_pages (ceil(count/0)), and Paginator([1,2,3,4,5], -1) produced num_pages == -5, with get_page(1) then raising an uncaught EmptyPage. Fixed by validating per_page in Paginator.init: coerce to int and raise ValueError("per_page must be at least 1, got {n}") when per_page < 1, before any state is set — closing the invariant at construction rather than relying on each caller (e.g. AdminListView) to clamp. Added plain/tests/public/test_paginator.py covering per_page=0, per_page=-1, and a normal positive case. Ran ./scripts/fix plain (ruff/oxlint/oxfmt/prettier, all clean) and the plain package's pytest suite (uv, with a bootstrapped uv since it wasn't preinstalled in this sandbox) — all 700 tests passed, including the 3 new ones. Full ./scripts/test could not run because this sandbox has neither Docker nor a local Postgres server for scripts/start-postgres to use; I instead ran uv run --isolated --package plain --with psycopg[binary] python -m pytest directly with a placeholder PLAIN_POSTGRES_URL, which was sufficient since none of the plain package's tests in this run touch a live database connection. Committed as a single commit on master.

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Opened by a PullApprove implementation run (implement-finding v4) for:

• PA-5 — Paginator accepts per_page <= 0 and crashes later instead of failing at construction

Merging this is what closes them as fixed.