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/database/database.go

@@ -2,6 +2,7 @@ package database
 
 import (
 	"log"
+	"time"
 
 	"gorm.io/driver/postgres"
 	"gorm.io/gorm"
@@ -24,6 +25,20 @@ func Connect(cfg *config.Config) (*gorm.DB, error) {
 	if err != nil {
 		return nil, err
 	}
+
+	// تنظیم connection pool؛ بدون این، پیش‌فرض Go بی‌نهایت اتصال باز می‌کنه که
+	// زیر بار بالا می‌تونه به‌راحتی خود Postgres رو با تعداد زیاد اتصال همزمان
+	// از پا در بیاره. MaxIdleConns هم اتصالات راکد رو محدود می‌کنه تا حافظه‌ی
+	// دیتابیس هدر نره.
+	sqlDB, err := db.DB()
+	if err != nil {
+		return nil, err
+	}
+	sqlDB.SetMaxOpenConns(cfg.DBMaxOpenConns)
+	sqlDB.SetMaxIdleConns(cfg.DBMaxIdleConns)
+	sqlDB.SetConnMaxLifetime(time.Duration(cfg.DBConnMaxLifetimeMins) * time.Minute)
+	sqlDB.SetConnMaxIdleTime(time.Duration(cfg.DBConnMaxIdleTimeMins) * time.Minute)
+
 	return db, nil
 }