feat: Phase 1 Group 6 — add account_id to maintenance_schedules

Primary backfill: tree_id → trees.account_id
Fallback: created_by → users.account_id (for is_default tree rows)

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

backend/tests/test_phase1_migrations.py

@@ -381,3 +381,42 @@ async def test_psa_member_mapping_account_id_matches_connection(test_db: AsyncSe
     )
     row = result.scalar_one()
     assert row.account_id == account.id
+
+
+# ── Group 6: Maintenance ──────────────────────────────────────────────────────
+
+@pytest.mark.asyncio
+async def test_maintenance_schedule_account_id_matches_tree(test_db: AsyncSession):
+    """maintenance_schedules.account_id must match the tree's account_id."""
+    from app.models.maintenance_schedule import MaintenanceSchedule
+
+    account, user = await _make_account_and_user(test_db, "ms1")
+    tree = Tree(
+        name="Maintenance Flow",
+        account_id=account.id,
+        author_id=user.id,
+        visibility="team",
+        tree_type="maintenance",
+        tree_structure={"id": "root", "type": "start", "children": []},
+        is_active=True,
+        status="published",
+    )
+    test_db.add(tree)
+    await test_db.flush()
+
+    schedule = MaintenanceSchedule(
+        tree_id=tree.id,
+        account_id=account.id,
+        created_by=user.id,
+        cron_expression="0 9 * * 1",
+        timezone="UTC",
+        is_active=True,
+    )
+    test_db.add(schedule)
+    await test_db.commit()
+
+    result = await test_db.execute(
+        select(MaintenanceSchedule).where(MaintenanceSchedule.id == schedule.id)
+    )
+    row = result.scalar_one()
+    assert row.account_id == account.id