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/alembic/versions/7f136778f5a8_add_account_id_maintenance.py

@@ -0,0 +1,62 @@
+"""add account_id to maintenance_schedules
+
+Revision ID: 7f136778f5a8
+Revises: 8aac5b372402
+Create Date: 2026-04-09 00:00:00.000000
+"""
+from typing import Sequence, Union
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = '7f136778f5a8'
+down_revision: Union[str, None] = '8aac5b372402'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column('maintenance_schedules',
+                  sa.Column('account_id', sa.UUID(), nullable=True))
+    op.create_foreign_key(
+        'fk_maintenance_schedules_account_id', 'maintenance_schedules', 'accounts',
+        ['account_id'], ['id'], ondelete='CASCADE',
+    )
+
+    # Primary: tree_id → trees.account_id (only where tree.account_id is NOT NULL)
+    op.execute("""
+        UPDATE maintenance_schedules ms
+        SET account_id = t.account_id
+        FROM trees t
+        WHERE ms.tree_id = t.id
+          AND t.account_id IS NOT NULL
+          AND ms.account_id IS NULL
+    """)
+
+    # Fallback: created_by → users.account_id (for is_default trees with NULL account_id)
+    op.execute("""
+        UPDATE maintenance_schedules ms
+        SET account_id = u.account_id
+        FROM users u
+        WHERE ms.created_by = u.id
+          AND u.account_id IS NOT NULL
+          AND ms.account_id IS NULL
+    """)
+
+    result = op.get_bind().execute(
+        sa.text("SELECT COUNT(*) FROM maintenance_schedules WHERE account_id IS NULL")
+    )
+    count = result.scalar()
+    if count > 0:
+        raise RuntimeError(
+            f"ROLLBACK: {count} maintenance_schedules rows have NULL account_id. "
+            "Check if created_by is NULL — those rows need manual resolution."
+        )
+
+    op.alter_column('maintenance_schedules', 'account_id', nullable=False)
+    op.create_index('ix_maintenance_schedules_account_id', 'maintenance_schedules', ['account_id'])
+
+
+def downgrade() -> None:
+    op.drop_index('ix_maintenance_schedules_account_id', table_name='maintenance_schedules')
+    op.drop_constraint('fk_maintenance_schedules_account_id', 'maintenance_schedules', type_='foreignkey')
+    op.drop_column('maintenance_schedules', 'account_id')