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/core/alerts.py

@@ -352,7 +352,8 @@ async def record_metrics(session: AsyncSession, rows: list[PluginMetric], phases
     for rule in rules:
         alert_state = states.get(rule.id)
         if alert_state is None:
-            logger.warning("No alert_state row for rule %s, skipping", rule.id)
+            # A restored rule (backups leave state out); alert_reconcile adds it.
+            logger.warning("No alert_state row for rule %s yet, skipping", rule.id)
             continue
         samples = sorted(by_key[(rule.source_module, rule.resource_name, rule.metric_name)],
                          key=lambda r: r.recorded_at)