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/app.py

@@ -429,9 +429,10 @@ def _register_core_tasks(app: Quart) -> None:
         await record_cleanup_secs(app, done["secs"])
 
     async def run_alert_reconcile():
-        from .alerts.service import reconcile_covering_rules
+        from .alerts.service import ensure_rule_states, reconcile_covering_rules
         from .core.alerts import check_silence, notify_after_maintenance
-        for step in (reconcile_covering_rules, check_silence, notify_after_maintenance):
+        for step in (reconcile_covering_rules, ensure_rule_states, check_silence,
+                     notify_after_maintenance):
             try:
                 await step(app)
             except Exception: