feat(PROJ-48): Audit-Log Unveränderbarkeit (Trigger, append-only Logfile, Healthcheck)

DB-Trigger audit_log_immutable verhindert UPDATE/DELETE auf audit_log,
zusätzliches append-only JSON-Lines-Logfile (audit.log_path) als
tamper-evident Backup, neuer Healthcheck-Prüfpunkt in archivmail status.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

config.test.yml

@@ -44,3 +44,9 @@ index:
 logging:
   path: /tmp/archivmail-test/logs
   level: debug
+
+audit:
+  # Append-only JSON-Lines Audit-Logdatei (PROJ-48).
+  # Default falls leer: /var/log/archivmail/audit.log
+  log_path: /tmp/archivmail-test/audit.log
+  retention_days: 0