fix: rebase migration onto correct alembic head

The pending_task_lane migration was branching from fb1481317ff6 which
already had a child (4f4137ce79e5). Fixes multiple-heads error.

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

backend/alembic/versions/fc01_add_pending_task_lane.py

@@ -9,7 +9,7 @@ import sqlalchemy as sa
 from sqlalchemy.dialects.postgresql import JSONB
 
 revision = "fc01a1b2c3d4"
-down_revision = "fb1481317ff6"
+down_revision = "4f4137ce79e5"
 branch_labels = None
 depends_on = None