Add production hardening: monitoring, backups, caching, and high-traffic resilience

- Recovery middleware (no more crash-on-panic), request metrics/monitoring
  store + admin API, in-memory TTL caching for read-heavy option/question/
  category lookups, gzip + rate-limiting middleware, sanitized chat message
  encoding, and pg_dump-based database backups with an admin API.
- Fix N+1 queries in the discover feed, candidate detail, and match list
  endpoints by batching selection/photo/user/last-message lookups into
  single queries instead of one-per-item loops.
- Tune DB connection pooling and add HTTP server timeouts + graceful
  shutdown so the API degrades gracefully instead of falling over under load.
- Fix a null (vs empty array) JSON response for candidates/matches without
  photos or interests, which crashed the web app's swipe card.

.gitignore

@@ -11,6 +11,9 @@
 # فایل‌های آپلودشده‌ی کاربران (نباید توی گیت باشن)
 /uploads/
 
+# بک‌آپ‌های دیتابیس (نباید توی گیت باشن؛ حجیم و شامل داده‌ی کاربرانه)
+/backups/
+
 # IDE
 .idea/
 .vscode/