Fix: Use WHERE instead of HAVING for min_count filter in list_tags()

HAVING requires GROUP BY; count_subq is a correlated scalar subquery, not
an aggregate, so PostgreSQL rejects it. WHERE works correctly and the
integration tests used SQLite which is permissive about this rule.

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

api/app/repositories/tag_repo.py

@@ -90,7 +90,7 @@ class TagRepository:
         if prefix:
             query = query.where(Tag.name.like(f"{prefix}%"))
         if min_count > 0:
-            query = query.having(count_subq >= min_count)
+            query = query.where(count_subq >= min_count)
 
         total_query = select(func.count()).select_from(query.subquery())
         total_result = await self._session.execute(total_query)