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/upload_service.go

@@ -16,6 +16,7 @@ import (
 	"strings"
 
 	"carrot_bbs/internal/pkg/s3"
+
 	_ "golang.org/x/image/bmp"
 	_ "golang.org/x/image/tiff"
 )
@@ -23,11 +24,11 @@ import (
 // UploadService 上传服务
 type UploadService struct {
 	s3Client    *s3.Client
-	userService *UserService
+	userService UserService
 }
 
 // NewUploadService 创建上传服务
-func NewUploadService(s3Client *s3.Client, userService *UserService) *UploadService {
+func NewUploadService(s3Client *s3.Client, userService UserService) *UploadService {
 	return &UploadService{
 		s3Client:    s3Client,
 		userService: userService,