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/recovery_middleware.go

@@ -0,0 +1,30 @@
+package middleware
+
+import (
+	"log"
+	"net/http"
+	"runtime/debug"
+
+	"github.com/gin-gonic/gin"
+
+	"dating-app/internal/utils"
+)
+
+// Recovery از کرش کردن کل پروسه به‌خاطر panic غیرمنتظره‌ی یک هندلر
+// (nil pointer، index out of range و...) جلوگیری می‌کنه: استک‌تریس رو لاگ
+// می‌کنه و به‌جای متن ساده‌ی پیش‌فرض Gin، همون envelope استاندارد JSON
+// پروژه (utils.Fail) رو برمی‌گردونه تا کلاینت‌ها فرمت یکسانی ببینن.
+func Recovery() gin.HandlerFunc {
+	return func(c *gin.Context) {
+		defer func() {
+			if r := recover(); r != nil {
+				log.Printf("[PANIC RECOVERED] %v\n%s", r, debug.Stack())
+				if !c.Writer.Written() {
+					utils.Fail(c, http.StatusInternalServerError, "خطای غیرمنتظره‌ای رخ داد؛ لطفاً دوباره امتحان کن", nil)
+				}
+				c.Abort()
+			}
+		}()
+		c.Next()
+	}
+}