Fix missing import for _log_admin_event in routes_customers

Added explicit import for _log_admin_event function which is needed for
customer export/import audit logging. Functions with underscore prefix
are not automatically imported by 'from .routes_shared import *'.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

containers/backupchecks/src/backend/app/main/routes_customers.py

@@ -1,4 +1,5 @@
 from .routes_shared import *  # noqa: F401,F403
+from .routes_shared import _log_admin_event
 
 # Explicit imports for robustness across mixed deployments.
 from datetime import datetime