refactor: rename bulk operations from /api/books/ to /api/media-items/

- Rename routes: /api/books/bulk-{delete,update} → /api/media-items/bulk-{delete,update}
- Rename route: /api/books/:uuid/download → /api/media-items/:uuid/download
- Update request fields: book_ids → media_item_ids
- Update response fields: success → deleted/updated
- Update result fields: book_id → media_item_id
- Update collection handler: success → added

This change improves API semantic correctness as the system handles
multiple media types (ebooks, comics, manga), not just books.

BREAKING CHANGE: All bulk operation endpoints and field names renamed

internal/handlers/collections.go

@@ -810,7 +810,7 @@ func (h *CollectionHandler) HandleBulkAddBooks(c echo.Context) error {
 	return c.JSON(http.StatusOK, map[string]interface{}{
 		"results": results,
 		"total":   len(results),
-		"success": successCount,
+		"added":   successCount,
 		"failed":  failedCount,
 	})
 }