feat(backup): a backup holds settings and keys only, for recovery (#5822)

#5822 "Backup contents: settings and keys only". The operator's ruling
(System 180, 2026-10-15) overturns 2026-10-12's "Config + audit + alerts":
a backup is for system recovery and carries what would otherwise have to
be set up again, with no logs or history.

- contents.py: audit_events, alert_events, alert_states, metric_anomalies,
  metric_anomaly_spans and user_invites move to EXCLUDED.
- Alert state is no longer restored, and evaluation skips a rule with no
  state row. ensure_rule_states (alerts/service.py) gives every
  single-resource rule lacking one an inactive row; alert_reconcile runs
  it each minute.
- Docs, the Backups page and the CLI help say what a backup holds now.
  CI's image check also finds the audit log empty after a restore.
- tests/integration/test_backup_restore_fixes.py is removed: its case (an
  alert event naming a left-out Ansible run) no longer arises. The SET NULL
  fix-up is covered by a unit test on synthetic tables.

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

steward/cli.py

@@ -65,8 +65,8 @@ def main(ctx: click.Context, config: str, host: str, port: int, debug: bool) ->
               help="The folder to write the backup file into.")
 @click.pass_context
 def backup_command(ctx: click.Context, out_dir: Path) -> None:
-    """Write a backup file of this Steward (configuration, audit log, alert
-    history and the secret key) into a folder, without sending it anywhere."""
+    """Write a backup file of this Steward (settings and the secret key: what
+    recovery needs, no logs or history) into a folder, without sending it anywhere."""
     from .backup.dump import BackupError, make_backup
     from .backup.job import migration_heads
     from .config import load_bootstrap