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.

.env.example

@@ -7,6 +7,11 @@ DB_USER=postgres
 DB_PASSWORD=postgres
 DB_NAME=dating_app
 DB_SSLMODE=disable
+# connection pool؛ زیر بار بالا این مقادیر رو با ظرفیت واقعی سرور Postgres تنظیم کن
+DB_MAX_OPEN_CONNS=25
+DB_MAX_IDLE_CONNS=10
+DB_CONN_MAX_LIFETIME_MINUTES=30
+DB_CONN_MAX_IDLE_TIME_MINUTES=5
 
 JWT_ACCESS_SECRET=test-access-secret
 JWT_REFRESH_SECRET=test-refresh-secret