docs(interview): rewrite all 8 interview posts as Junior→Senior Q&A series (en+vi)

[hungpt99-dev]

What

Full rewrite of all 8 interview posts (× en + vi = 16 files) into a coherent Java Interview Prep #1–#8 series. Each post replaces the old deep-dive body with fresh, Q&A-driven prose structured as a Junior → Mid → Senior progression, plus a per-level Self-check.

Series order

1. Java Core (JVM, GC, Concurrency)
2. OOP & Design Principles
3. Spring Boot
4. Database & SQL
5. Apache Kafka
6. Microservices
7. System Design
8. Senior Mindset & Behavioral

Series titles (new)

• EN: Java Interview Prep #N: <Topic> — Junior to Senior
• VI: Ôn thi Java #N: <Topic> — Junior đến Senior (vi is the default locale, kept rich — not simplified)

Preserved (AGENTS.md hard rules)

• pubDatetime unchanged on every file
• frontmatter keys within schema; tags/featured unchanged
• bilingual EN+VI pairing, same depth + code examples, faithful translation

Verification

• pnpm format clean
• pnpm build (Astro) exit 0 — 52 pages indexed, 12,752 words
• 8 atomic commits, one per topic

This is additive-in-spirit content work; no code/layout changes.

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