Route deletion failed with two integrity errors:
1. route_encounters had no cascade, so SQLAlchemy tried to NULL
the non-nullable route_id instead of deleting the rows
2. boss_battles.after_route_id referenced the route being deleted
Added cascade="all, delete-orphan" to Route.route_encounters and
nulled out boss battle after_route_id references before deletion.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
