refactor: introduce Wire dependency injection and interface-based architecture

- Add Google Wire for compile-time dependency injection
- Replace concrete service types with interfaces across handlers
- Remove global state (DB, cache) in favor of constructor injection
- Split monolithic files into focused modules:
  - config: separate files for each config domain
  - dto: converters split by domain (user, post, message, group)
  - cache: separate metrics.go and redis_cache.go
- Introduce unified apperrors package with string-based error codes
- Add transaction support with context-aware repository methods
- Upgrade golang.org/x/crypto from 0.17.0 to 0.26.0

internal/service/system_message_service.go

@@ -42,13 +42,14 @@ func NewSystemMessageService(
 	pushService PushService,
 	userRepo *repository.UserRepository,
 	postRepo *repository.PostRepository,
+	cacheBackend cache.Cache,
 ) SystemMessageService {
 	return &systemMessageServiceImpl{
 		notifyRepo:  notifyRepo,
 		pushService: pushService,
 		userRepo:    userRepo,
 		postRepo:    postRepo,
-		cache:       cache.GetCache(),
+		cache:       cacheBackend,
 	}
 }