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/chat_service.go

@@ -72,6 +72,7 @@ func NewChatService(
 	userRepo *repository.UserRepository,
 	sensitive SensitiveService,
 	sseHub *sse.Hub,
+	cacheBackend cache.Cache,
 ) ChatService {
 	// 创建适配器
 	convRepoAdapter := cache.NewConversationRepositoryAdapter(repo)
@@ -79,7 +80,7 @@ func NewChatService(
 
 	// 创建会话缓存
 	conversationCache := cache.NewConversationCache(
-		cache.GetCache(),
+		cacheBackend,
 		convRepoAdapter,
 		msgRepoAdapter,
 		cache.DefaultConversationCacheSettings(),