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/message_service.go

@@ -37,14 +37,14 @@ type MessageService struct {
 }
 
 // NewMessageService 创建消息服务
-func NewMessageService(db *gorm.DB, messageRepo *repository.MessageRepository) *MessageService {
+func NewMessageService(db *gorm.DB, messageRepo *repository.MessageRepository, cacheBackend cache.Cache) *MessageService {
 	// 创建适配器
 	convRepoAdapter := cache.NewConversationRepositoryAdapter(messageRepo)
 	msgRepoAdapter := cache.NewMessageRepositoryAdapter(messageRepo)
 
 	// 创建会话缓存
 	conversationCache := cache.NewConversationCache(
-		cache.GetCache(),
+		cacheBackend,
 		convRepoAdapter,
 		msgRepoAdapter,
 		cache.DefaultConversationCacheSettings(),
@@ -54,7 +54,7 @@ func NewMessageService(db *gorm.DB, messageRepo *repository.MessageRepository) *
 		db:                db,
 		messageRepo:       messageRepo,
 		conversationCache: conversationCache,
-		baseCache:         cache.GetCache(),
+		baseCache:         cacheBackend,
 	}
 }