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/lib/server/db/repository.ts

@@ -158,6 +158,7 @@ export interface TagRepository {
 	getById(id: string): Tag | undefined;
 	getByEntryId(entryId: string): Tag[];
 	updateEntryTags(entryId: string, tagNames: string[]): void;
+	cleanupOrphanedTags(): number;
 }
 
 class SQLiteTagRepository implements TagRepository {
@@ -215,6 +216,15 @@ class SQLiteTagRepository implements TagRepository {
 			db.insert(entryTags).values({ entryId, tagId: tag.id }).run();
 		}
 	}
+
+	cleanupOrphanedTags(): number {
+		// Delete tags that are not associated with any entry
+		const result = db.run(sql`
+			DELETE FROM tags
+			WHERE id NOT IN (SELECT DISTINCT tag_id FROM entry_tags)
+		`);
+		return result.changes;
+	}
 }
 
 export const tagRepository: TagRepository = new SQLiteTagRepository();