fix: AI Assistant import + admin role in seed_admin

- ai_assistant/routes.py: add missing apply_visibility_filter import
- seed_admin.py: set role="admin" on UserTenant (was defaulting to viewer)

app/plugins/builtins/ai_assistant/routes.py

@@ -13,6 +13,7 @@ from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.core.db import get_db, set_tenant_context
+from app.core.visibility import apply_visibility_filter
 from app.deps import get_current_user, require_permission
 from app.plugins.builtins.ai_assistant.models import (
     AIAgent,