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/monitoring_handler.go

@@ -0,0 +1,35 @@
+package handler
+
+import (
+	"net/http"
+	"strconv"
+
+	"github.com/gin-gonic/gin"
+
+	"dating-app/internal/monitoring"
+	"dating-app/internal/utils"
+)
+
+type MonitoringHandler struct {
+	store *monitoring.Store
+}
+
+func NewMonitoringHandler(store *monitoring.Store) *MonitoringHandler {
+	return &MonitoringHandler{store: store}
+}
+
+// Summary GET /admin/monitoring/summary
+func (h *MonitoringHandler) Summary(c *gin.Context) {
+	utils.Success(c, http.StatusOK, "", h.store.Summary())
+}
+
+// RecentRequests GET /admin/monitoring/requests?limit=100
+func (h *MonitoringHandler) RecentRequests(c *gin.Context) {
+	limit := 100
+	if raw := c.Query("limit"); raw != "" {
+		if n, err := strconv.Atoi(raw); err == nil && n > 0 && n <= 1000 {
+			limit = n
+		}
+	}
+	utils.Success(c, http.StatusOK, "", h.store.Recent(limit))
+}