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/comment_service.go

@@ -24,12 +24,12 @@ type CommentService struct {
 }
 
 // NewCommentService 创建评论服务
-func NewCommentService(commentRepo *repository.CommentRepository, postRepo *repository.PostRepository, systemMessageService SystemMessageService, gorseClient gorse.Client, postAIService *PostAIService) *CommentService {
+func NewCommentService(commentRepo *repository.CommentRepository, postRepo *repository.PostRepository, systemMessageService SystemMessageService, gorseClient gorse.Client, postAIService *PostAIService, cacheBackend cache.Cache) *CommentService {
 	return &CommentService{
 		commentRepo:          commentRepo,
 		postRepo:             postRepo,
 		systemMessageService: systemMessageService,
-		cache:                cache.GetCache(),
+		cache:                cacheBackend,
 		gorseClient:          gorseClient,
 		postAIService:        postAIService,
 	}