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/notification_service.go

@@ -5,6 +5,7 @@ import (
 	"time"
 
 	"carrot_bbs/internal/cache"
+	apperrors "carrot_bbs/internal/errors"
 	"carrot_bbs/internal/model"
 	"carrot_bbs/internal/repository"
 )
@@ -23,10 +24,10 @@ type NotificationService struct {
 }
 
 // NewNotificationService 创建通知服务
-func NewNotificationService(notificationRepo *repository.NotificationRepository) *NotificationService {
+func NewNotificationService(notificationRepo *repository.NotificationRepository, cacheBackend cache.Cache) *NotificationService {
 	return &NotificationService{
 		notificationRepo: notificationRepo,
-		cache:            cache.GetCache(),
+		cache:            cacheBackend,
 	}
 }
 
@@ -166,4 +167,4 @@ func (s *NotificationService) ClearAllNotifications(ctx context.Context, userID
 }
 
 // 错误定义
-var ErrUnauthorizedNotification = &ServiceError{Code: 403, Message: "unauthorized to delete this notification"}
+var ErrUnauthorizedNotification = apperrors.ErrUnauthorizedNotification