feat: v1.2.0 — validation hardening, unit tests, quality improvements
- Fix task status enum mismatch (blocked → quality_review)
- Add 12 Zod schemas for all unvalidated mutation routes
- Apply validateBody() across 11 API route handlers
- Add readLimiter (120/min) for GET-heavy endpoints
- Extend heavyLimiter to search, backup, cleanup routes
- Add security headers (X-Content-Type-Options, X-Frame-Options, Referrer-Policy)
- Fill auth test stubs with real assertions (safeCompare, requireRole)
- Add validation, rate-limit, and db-helpers unit test suites (60 tests total)
- Replace as-any casts with typed interfaces (SessionQueryRow, UserQueryRow, CountRow)
- Bump version to 1.2.0, add CHANGELOG.md, update README roadmap
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