feat: implement multiple folder ebook scanner and API handlers

- Create EbookScanner service with multiple folder support
- Update scanner to handle array of folder paths instead of single path
- Add EPUB metadata extraction for rich ebook information
- Implement folder monitoring with fsnotify for real-time updates
- Add API endpoints for folder management:
  - POST /api/auth/ebook-folders (add folder)
  - GET /api/auth/ebook-folders (list folders)
  - DELETE /api/auth/ebook-folders/:folderPath (remove folder)
- Update scanner endpoints to accept folder_paths array
- Add go-epub and fsnotify dependencies for metadata extraction and file watching

backend/internal/handlers/auth.go

@@ -315,6 +315,99 @@ func (h *AuthHandler) ListUsers(c echo.Context) error {
 	return c.JSON(http.StatusOK, userList)
 }
 
+type AddEbookFolderRequest struct {
+	FolderPath string `json:"folder_path" validate:"required"`
+}
+
+type EbookFolderResponse struct {
+	ID         string `json:"id"`
+	UserID     string `json:"user_id"`
+	FolderPath string `json:"folder_path"`
+	CreatedAt  string `json:"created_at"`
+}
+
+// AddEbookFolder handles POST /api/auth/ebook-folders
+func (h *AuthHandler) AddEbookFolder(c echo.Context) error {
+	userID := c.Get("user_id").(string)
+	userUUID, err := uuid.Parse(userID)
+	if err != nil {
+		return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid user id"})
+	}
+
+	var req AddEbookFolderRequest
+	if err := c.Bind(&req); err != nil {
+		return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid request"})
+	}
+	if err := c.Validate(&req); err != nil {
+		return c.JSON(http.StatusBadRequest, map[string]string{"error": err.Error()})
+	}
+
+	folder, err := h.db.AddUserEbookFolder(c.Request().Context(), database.AddUserEbookFolderParams{
+		UserID:     pgtype.UUID{Bytes: userUUID, Valid: true},
+		FolderPath: req.FolderPath,
+	})
+	if err != nil {
+		return c.JSON(http.StatusInternalServerError, map[string]string{"error": err.Error()})
+	}
+
+	return c.JSON(http.StatusCreated, EbookFolderResponse{
+		ID:         uuid.UUID(folder.ID.Bytes).String(),
+		UserID:     uuid.UUID(folder.UserID.Bytes).String(),
+		FolderPath: folder.FolderPath,
+		CreatedAt:  folder.CreatedAt.Time.Format("2006-01-02T15:04:05Z07:00"),
+	})
+}
+
+// GetEbookFolders handles GET /api/auth/ebook-folders
+func (h *AuthHandler) GetEbookFolders(c echo.Context) error {
+	userID := c.Get("user_id").(string)
+	userUUID, err := uuid.Parse(userID)
+	if err != nil {
+		return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid user id"})
+	}
+
+	folders, err := h.db.GetUserEbookFolders(c.Request().Context(), pgtype.UUID{Bytes: userUUID, Valid: true})
+	if err != nil {
+		return c.JSON(http.StatusInternalServerError, map[string]string{"error": err.Error()})
+	}
+
+	var response []EbookFolderResponse
+	for _, folder := range folders {
+		response = append(response, EbookFolderResponse{
+			ID:         uuid.UUID(folder.ID.Bytes).String(),
+			UserID:     uuid.UUID(folder.UserID.Bytes).String(),
+			FolderPath: folder.FolderPath,
+			CreatedAt:  folder.CreatedAt.Time.Format("2006-01-02T15:04:05Z07:00"),
+		})
+	}
+
+	return c.JSON(http.StatusOK, response)
+}
+
+// DeleteEbookFolder handles DELETE /api/auth/ebook-folders/:folderPath
+func (h *AuthHandler) DeleteEbookFolder(c echo.Context) error {
+	userID := c.Get("user_id").(string)
+	userUUID, err := uuid.Parse(userID)
+	if err != nil {
+		return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid user id"})
+	}
+
+	folderPath := c.Param("folderPath")
+	if folderPath == "" {
+		return c.JSON(http.StatusBadRequest, map[string]string{"error": "folder path is required"})
+	}
+
+	err = h.db.DeleteUserEbookFolder(c.Request().Context(), database.DeleteUserEbookFolderParams{
+		UserID:     pgtype.UUID{Bytes: userUUID, Valid: true},
+		FolderPath: folderPath,
+	})
+	if err != nil {
+		return c.JSON(http.StatusInternalServerError, map[string]string{"error": err.Error()})
+	}
+
+	return c.JSON(http.StatusOK, map[string]string{"message": "ebook folder removed"})
+}
+
 func (h *AuthHandler) generateJWT(userID string) (string, error) {
 	claims := jwtgo.MapClaims{
 		"user_id": userID,