feat: add browsable question bank

[369pro]

What changed

• add a tenant-scoped, paginated question-bank browse API backed by active MySQL rows
• turn “我的题库” into a searchable three-pane library with source, difficulty and type filters
• show full questions, reference answers, skills, source files and index versions after upload
• document the behavior in the spec and README

Why

Uploading a question bank previously produced only a success banner, so users could not inspect what had been parsed or indexed. The new library makes uploaded content visible and verifiable while preserving user isolation.

Validation

• node --check src/interview_agent/web/app.js
• uv run ruff check .
• uv run mypy src
• uv run pytest tests/unit -q — 58 passed
• RUN_INTEGRATION=1 uv run pytest tests/integration -q — 8 passed
• secret scan passed
• Docker services rebuilt and healthy at http://127.0.0.1:9092/
• Playwright verified upload, full answer detail, search, source/type filters, pagination, refresh persistence, empty state and cross-user isolation