chore(backend): implement high-priority improvements from code review
- Configuration: dotenv support, env vars for PORT, DB_PATH, CORS_ORIGIN, rate limits
- Security: API key auth for write endpoints, rate limiting on import, configurable CORS, image URL normalization
- Reliability: DB transactions for create/update, dirty flag for saves, foreign key enforcement (PRAGMA), duplicate detection O(1) optimization
- Features: health check endpoint (/api/health)
- Bugfix: corrected tag assignment/removal routes (param order)
- Testing: added tests for PUT/DELETE recipes, tag CRUD and assignment, enabled foreign keys in tests
- All 46 backend tests passing
Closes #<ticket-if-any>
8b729d7fc4