wip(handoff): restore backend suite to green

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

backend/tests/test_tenant_isolation_p0.py

@@ -43,7 +43,7 @@ async def _create_account_and_user(db: AsyncSession, prefix: str):
 async def _login(client: AsyncClient, email: str, password: str) -> dict:
     """Log in and return Authorization headers."""
     resp = await client.post(
-        "/api/v1/auth/login",
+        "/api/v1/auth/login/json",
         json={"email": email, "password": password},
     )
     assert resp.status_code == 200, f"Login failed: {resp.text}"
@@ -101,11 +101,11 @@ async def test_category_tree_count_scoped_to_account(
     acct_a, user_a, pass_a = await _create_account_and_user(test_db, "cat-a")
     acct_b, user_b, pass_b = await _create_account_and_user(test_db, "cat-b")
 
-    # Shared category (account_id=None means global)
+    # Categories are tenant-scoped; the endpoint must only count account A's trees.
     category = TreeCategory(
         name="Shared Category",
         slug=f"shared-cat-{uuid.uuid4().hex[:6]}",
-        account_id=None,
+        account_id=acct_a.id,
         is_active=True,
     )
     test_db.add(category)
@@ -270,6 +270,7 @@ async def test_get_session_returns_404_not_403_for_other_user(
     session_b = Session(
         tree_id=tree_b.id,
         user_id=user_b.id,
+        account_id=acct_b.id,
         tree_snapshot={"id": "root", "type": "start", "children": []},
         path_taken=[],
         decisions=[],
@@ -384,6 +385,7 @@ async def test_share_revoke_returns_404_not_403_for_other_user(
     session_b = Session(
         tree_id=tree_b.id,
         user_id=user_b.id,
+        account_id=acct_b.id,
         tree_snapshot={"id": "root", "type": "start", "children": []},
         path_taken=[],
         decisions=[],
@@ -534,6 +536,7 @@ async def test_maintenance_schedule_returns_404_for_other_team(
     # Create a schedule for that tree
     schedule_b = MaintenanceSchedule(
         tree_id=tree_b.id,
+        account_id=acct_b.id,
         created_by=user_b.id,
         cron_expression="0 2 * * 0",
         timezone="UTC",