feat: 书库导入导出/图片变体/书籍搜索/小组件运行时等

新增：
- 书库导入导出（library_import/library_export）及测试
- 图片变体生成（image_variants）与响应式图片（responsiveImage）
- 书籍搜索（bookSearch）+ BookSearch/LibrarySearchGrid 组件
- 小组件运行时（widgetRuntime）与静态检查（widgetLint）
- 上传缓存中间件（upload_cache）与字体 CSS 提取脚本

其它：
- 后端 handlers/services 全量调整
- 前端页面、组件、库函数与配置更新

backend/service/upload.go

@@ -238,7 +238,7 @@ func (s *UploadService) transcodeImageToWebP(tmp, name, ext string) (string, str
 }
 
 // SaveImage 流式保存帖子插图：校验格式/大小/尺寸 → JPEG/PNG 转 WebP → 落盘 → 写 attachments（kind=image）
-func (s *UploadService) SaveImage(userID uint, src io.Reader) (*model.Attachment, error) {
+func (s *UploadService) SaveImage(userID uint, src io.Reader, source string) (*model.Attachment, error) {
 	if src == nil {
 		return nil, errors.New("文件为空")
 	}
@@ -354,6 +354,7 @@ func (s *UploadService) SaveImage(userID uint, src io.Reader) (*model.Attachment
 	att := &model.Attachment{
 		UserID: userID,
 		Kind:   model.AttachmentKindImage,
+		Source: normalizeImageSource(source),
 		URL:    "/uploads/images/" + storeName,
 		MIME:   storeMime,
 		Size:   int(storeSize),
@@ -585,9 +586,20 @@ func (s *UploadService) CopyBackgroundFromMedia(userID, attachmentID uint) (stri
 	return s.SaveBackground(f)
 }
 
+// normalizeImageSource 把外部传入的图片来源收敛到白名单，未知值回退为 post
+func normalizeImageSource(s string) string {
+	switch s {
+	case model.AttachmentSourceLibraryCover, model.AttachmentSourceLibraryContent:
+		return s
+	default:
+		return model.AttachmentSourcePost
+	}
+}
+
 // CopyImageFromMedia 把当前用户媒体库里的一张图复制一份新插图（落 uploads/images 并记一条本人附件）
 // 用于书籍封面等长期引用场景：与原图解耦，原图删除后副本仍可用
-func (s *UploadService) CopyImageFromMedia(userID, attachmentID uint) (*model.Attachment, error) {
+// source 决定副本的来源标记（如 library_cover）
+func (s *UploadService) CopyImageFromMedia(userID, attachmentID uint, source string) (*model.Attachment, error) {
 	var att model.Attachment
 	if err := s.db.Where("id = ? AND user_id = ?", attachmentID, userID).First(&att).Error; err != nil {
 		return nil, errors.New("图片不存在或不属于你")
@@ -600,7 +612,7 @@ func (s *UploadService) CopyImageFromMedia(userID, attachmentID uint) (*model.At
 			return nil, e
 		}
 		defer r.Close()
-		return s.SaveImage(userID, r)
+		return s.SaveImage(userID, r, source)
 	}
 	abs, ok := s.safeUploadPath(att.URL)
 	if !ok {
@@ -611,7 +623,7 @@ func (s *UploadService) CopyImageFromMedia(userID, attachmentID uint) (*model.At
 		return nil, errors.New("读取图片失败")
 	}
 	defer f.Close()
-	return s.SaveImage(userID, f)
+	return s.SaveImage(userID, f, source)
 }
 
 // BackgroundFileExists 确认 URL 对应文件在 backgrounds 目录内
@@ -667,6 +679,15 @@ func (s *UploadService) ListMedia(userID uint) ([]model.Attachment, error) {
 	return list, err
 }
 
+// FindMediaBySource 按来源查本人附件（存在返回记录，不存在返回 error）
+func (s *UploadService) FindMediaBySource(userID, attachmentID uint, source string) (*model.Attachment, error) {
+	var att model.Attachment
+	if err := s.db.Where("id = ? AND user_id = ? AND source = ?", attachmentID, userID, source).First(&att).Error; err != nil {
+		return nil, err
+	}
+	return &att, nil
+}
+
 // ErrAttachmentInUse 图片正被帖子内容引用，不可物理删除
 var ErrAttachmentInUse = errors.New("该图片已被帖子使用，无法删除")