Add database-backed bookmarks via GraphQL

Replace hardcoded bookmarks in the frontend with a GORM-backed Bookmark
model exposed through GraphQL query and admin-only create/delete mutations.
Frontend groups bookmarks by category dynamically from the store.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/services/database.go

@@ -39,6 +39,7 @@ func migrateDatabase(db *gorm.DB) error {
 		&models.Rowing{},
 		&models.Message{},
 		&models.JobApplication{},
+		&models.Bookmark{},
 	)
 	if err != nil {
 		return err