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/post_handler.go

@@ -15,12 +15,12 @@ import (
 
 // PostHandler 帖子处理器
 type PostHandler struct {
-	postService *service.PostService
-	userService *service.UserService
+	postService service.PostService
+	userService service.UserService
 }
 
 // NewPostHandler 创建帖子处理器
-func NewPostHandler(postService *service.PostService, userService *service.UserService) *PostHandler {
+func NewPostHandler(postService service.PostService, userService service.UserService) *PostHandler {
 	return &PostHandler{
 		postService: postService,
 		userService: userService,