feat: auto-cleanup orphaned tags when removed from entries

Tags that are no longer associated with any entry are automatically
deleted from the database when a tag is removed from an entry.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/routes/+page.server.ts

@@ -342,6 +342,9 @@ export const actions: Actions = {
 		);
 		tagRepository.updateEntryTags(id, updatedTags);
 
+		// Clean up orphaned tags
+		tagRepository.cleanupOrphanedTags();
+
 		return { success: true };
 	}
 };