fix: shared filter returns all when user has no team

When user.team_id is None, `WHERE team_id == None` matches all
personal templates. Return empty set instead when user has no team.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/api/endpoints/scripts.py

@@ -5,7 +5,7 @@ import re
 
 from fastapi import APIRouter, Depends, HTTPException, Query, status
 from sqlalchemy.ext.asyncio import AsyncSession
-from sqlalchemy import select, func, or_
+from sqlalchemy import select, func, or_, literal
 
 from app.core.database import get_db
 from app.api.deps import get_current_active_user
@@ -122,7 +122,10 @@ async def list_templates(
         query = query.where(ScriptTemplate.created_by == current_user.id)
 
     if shared:
-        query = query.where(ScriptTemplate.team_id == current_user.team_id)
+        if current_user.team_id is None:
+            query = query.where(literal(False))
+        else:
+            query = query.where(ScriptTemplate.team_id == current_user.team_id)
 
     result = await db.execute(query.order_by(ScriptTemplate.name))
     templates = result.scalars().all()