wip(handoff): restore backend suite to green

Co-Authored-By: Codex <noreply@openai.com>

backend/tests/test_tree_sharing.py

@@ -4,6 +4,7 @@ from datetime import datetime, timezone, timedelta
 from httpx import AsyncClient
 from uuid import uuid4
 
+from app.models.account import Account
 from app.models.tree import Tree
 from app.models.tree_share import TreeShare
 from app.models.user import User
@@ -287,13 +288,17 @@ class TestTreeSharing:
 @pytest.mark.asyncio
 async def test_migration_defaults_visibility_to_team(test_db):
     """Test that existing trees default to 'team' visibility after migration."""
+    account = Account(name="Migration Default Test", display_code=uuid4().hex[:8])
+    test_db.add(account)
+    await test_db.flush()
+
     # Create a tree without specifying visibility
     tree = Tree(
         name="Old Tree",
         description="Created before migration",
         tree_structure={"id": "root", "type": "decision", "question": "Test?", "children": []},
         author_id=None,
-        account_id=None
+        account_id=account.id
     )
     test_db.add(tree)
     await test_db.commit()