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/middleware/metrics_middleware.go

@@ -0,0 +1,44 @@
+package middleware
+
+import (
+	"time"
+
+	"github.com/gin-gonic/gin"
+
+	"dating-app/internal/models"
+	"dating-app/internal/monitoring"
+)
+
+// Metrics هر درخواست رو (متد، مسیر، کد پاسخ، مدت زمان) توی Store ثبت می‌کنه.
+// باید قبل از Recovery ثبت بشه (بیرونی‌تر) تا حتی درخواست‌هایی که پنیک می‌کنن
+// و توسط Recovery مهار می‌شن هم توی آمار لحاظ بشن.
+func Metrics(store *monitoring.Store) gin.HandlerFunc {
+	return func(c *gin.Context) {
+		start := time.Now()
+
+		defer func() {
+			path := c.FullPath()
+			if path == "" {
+				path = c.Request.URL.Path
+			}
+
+			rec := monitoring.RequestRecord{
+				Time:       start,
+				Method:     c.Request.Method,
+				Path:       path,
+				StatusCode: c.Writer.Status(),
+				DurationMs: float64(time.Since(start).Microseconds()) / 1000.0,
+				ClientIP:   c.ClientIP(),
+			}
+			if userVal, exists := c.Get(ContextUserKey); exists {
+				if user, ok := userVal.(*models.User); ok {
+					id := user.ID
+					rec.UserID = &id
+				}
+			}
+			store.Record(rec)
+		}()
+
+		c.Next()
+	}
+}