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/rate_limit_middleware_test.go

@@ -0,0 +1,74 @@
+package middleware
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/gin-gonic/gin"
+)
+
+func newTestEngine(mw gin.HandlerFunc) *gin.Engine {
+	gin.SetMode(gin.TestMode)
+	engine := gin.New()
+	engine.GET("/x", mw, func(c *gin.Context) {
+		c.JSON(http.StatusOK, gin.H{"ok": true})
+	})
+	return engine
+}
+
+func TestRateLimiter_AllowsWithinLimit(t *testing.T) {
+	engine := newTestEngine(RateLimiter(3, 1))
+
+	for i := 0; i < 3; i++ {
+		req := httptest.NewRequest(http.MethodGet, "/x", nil)
+		req.RemoteAddr = "1.2.3.4:1111"
+		w := httptest.NewRecorder()
+		engine.ServeHTTP(w, req)
+		if w.Code != http.StatusOK {
+			t.Fatalf("request %d: expected 200, got %d", i+1, w.Code)
+		}
+	}
+}
+
+func TestRateLimiter_BlocksOverLimit(t *testing.T) {
+	engine := newTestEngine(RateLimiter(2, 0)) // بدون شارژ دوباره، فقط ۲ تا مجازه
+
+	for i := 0; i < 2; i++ {
+		req := httptest.NewRequest(http.MethodGet, "/x", nil)
+		req.RemoteAddr = "5.6.7.8:2222"
+		w := httptest.NewRecorder()
+		engine.ServeHTTP(w, req)
+		if w.Code != http.StatusOK {
+			t.Fatalf("request %d: expected 200, got %d", i+1, w.Code)
+		}
+	}
+
+	req := httptest.NewRequest(http.MethodGet, "/x", nil)
+	req.RemoteAddr = "5.6.7.8:2222"
+	w := httptest.NewRecorder()
+	engine.ServeHTTP(w, req)
+	if w.Code != http.StatusTooManyRequests {
+		t.Fatalf("expected 429 on 3rd request, got %d", w.Code)
+	}
+}
+
+func TestRateLimiter_SeparateBucketsPerIP(t *testing.T) {
+	engine := newTestEngine(RateLimiter(1, 0))
+
+	req1 := httptest.NewRequest(http.MethodGet, "/x", nil)
+	req1.RemoteAddr = "9.9.9.9:1"
+	w1 := httptest.NewRecorder()
+	engine.ServeHTTP(w1, req1)
+	if w1.Code != http.StatusOK {
+		t.Fatalf("expected 200 for first IP, got %d", w1.Code)
+	}
+
+	req2 := httptest.NewRequest(http.MethodGet, "/x", nil)
+	req2.RemoteAddr = "8.8.8.8:1"
+	w2 := httptest.NewRecorder()
+	engine.ServeHTTP(w2, req2)
+	if w2.Code != http.StatusOK {
+		t.Fatalf("expected 200 for a different IP (separate bucket), got %d", w2.Code)
+	}
+}