fix(backup): refuse another schema before reading the dump; tests read settings directly (#5815)

- restore_in_place checks the backup's migration heads before pg_restore
  writes anything, so a backup from another schema is refused at once.
- The integration tests read their own setting rows directly:
  get_all_settings knows only Steward's own keys.
- check.py: drop an import left unused by the move to pgdata.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

steward/backup/check.py

@@ -35,7 +35,7 @@ from sqlalchemy.ext.asyncio import create_async_engine
 
 from .contents import (ACTIVITY, ACTIVITY_EVERYWHERE, AREAS, EXCLUDED, HAND_SET_ONLY,
                        STATE_SETTINGS)
-from .dump import BackupError, read_backup
+from .dump import read_backup
 from .pgdata import async_url as _async_url
 from .pgdata import copy_rows, restore_script, script_tables