优化存储逻辑：为 WebDAV、Local 和 S3 增加 Exists 方法；调整图片处理逻辑以避免重复存储变体；新增调试日志以便于排查问题

internal/storage/storage.go

@@ -17,6 +17,7 @@ type Storage interface {
 	Get(ctx context.Context, key string) (io.ReadCloser, string, error)
 	Delete(ctx context.Context, key string) error
 	PublicURL(key string) (string, bool)
+	Exists(ctx context.Context, key string) (bool, error)
 }
 
 var GlobalStorage Storage