9bb89ee62f
- schema.sql: all user_id FKs now ON DELETE SET NULL (was RESTRICT) - migrate_user_fk_set_null.py: drop+readd constraints on existing DBs - entrypoint.sh: runs migration automatically on startup (non-fatal) - db.py: rollback moved before FK check (cleanup) Deleting a user now nullifies their references in audit_log, project_context, context_files, change_requests, reviews instead of blocking with a 409.