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/handler/message_handler.go

@@ -20,13 +20,13 @@ import (
 type MessageHandler struct {
 	chatService    service.ChatService
 	messageService *service.MessageService
-	userService    *service.UserService
+	userService    service.UserService
 	groupService   service.GroupService
 	sseHub         *sse.Hub
 }
 
 // NewMessageHandler 创建消息处理器
-func NewMessageHandler(chatService service.ChatService, messageService *service.MessageService, userService *service.UserService, groupService service.GroupService, sseHub *sse.Hub) *MessageHandler {
+func NewMessageHandler(chatService service.ChatService, messageService *service.MessageService, userService service.UserService, groupService service.GroupService, sseHub *sse.Hub) *MessageHandler {
 	return &MessageHandler{
 		chatService:    chatService,
 		messageService: messageService,