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.

internal/handler/chat_handler.go

@@ -81,6 +81,10 @@ func (h *ChatHandler) handleError(c *gin.Context, err error) {
 	switch {
 	case errors.Is(err, service.ErrNotMatchMember):
 		utils.Fail(c, http.StatusForbidden, err.Error(), nil)
+	case errors.Is(err, utils.ErrInvalidMessageEncoding),
+		errors.Is(err, utils.ErrMessageEmpty),
+		errors.Is(err, utils.ErrMessageTooLong):
+		utils.Fail(c, http.StatusBadRequest, err.Error(), nil)
 	default:
 		utils.Fail(c, http.StatusNotFound, "گفتگو یافت نشد", nil)
 	}