fix: handler ISBN field type corrections

- Fix CreateMediaItem ISBN field to use pgtype.Text wrapper
- Fix UpdateMediaItem to use correct Isbn field name
- Resolve type mismatch between request and database params

Resolves compilation errors in media item handlers

internal/handlers/ebook.go

@@ -795,7 +795,7 @@ func (h *Handler) CreateMediaItem(c echo.Context) error {
 		LibraryID:      pgtype.UUID{Bytes: req.LibraryID, Valid: true},
 		Title:          req.Title,
 		Author:         pgtype.Text{String: req.Author, Valid: req.Author != ""},
-		Isbn:           req.ISBN,
+		Isbn:           pgtype.Text{String: req.ISBN, Valid: req.ISBN != ""},
 		Description:    pgtype.Text{String: req.Description, Valid: req.Description != ""},
 		FilePath:       req.FilePath,
 		FileSize:       pgtype.Int8{Int64: req.FileSize, Valid: req.FileSize > 0},
@@ -866,7 +866,7 @@ func (h *Handler) UpdateMediaItem(c echo.Context) error {
 		ID:             pgtype.UUID{Bytes: mediaUUID, Valid: true},
 		Title:          req.Title,
 		Author:         pgtype.Text{String: req.Author, Valid: req.Author != ""},
-		Isbn:           req.ISBN,
+		Isbn:           pgtype.Text{String: req.ISBN, Valid: req.ISBN != ""},
 		Description:    pgtype.Text{String: req.Description, Valid: req.Description != ""},
 		CoverImagePath: pgtype.Text{String: req.CoverImagePath, Valid: req.CoverImagePath != ""},
 		Series:         pgtype.Text{String: req.Series, Valid: req.Series != ""},