fix(availability): translate status messages, return stored admin reason from API

- Add AvailabilityStatusDetails record and GetStatusDetailsAsync() to
  IAvailabilityService; implement in PersistentAvailabilityService so the
  admin-set reason stored in the database is returned alongside the status
- AvailabilityController.GetStatus() now returns the stored message instead
  of hardcoded English strings
- Add messageAvailable / messageMaintenance / messageUnavailable translation
  keys in en + nl so default messages are fully translatable
- AvailabilityStatusBadge: Available always shows the translated default;
  Maintenance and Unavailable show the custom admin reason when set,
  otherwise fall back to the translated default

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

frontend/src/i18n/locales/en/translation.json

@@ -66,6 +66,9 @@
         "available": "Available",
         "maintenance": "Maintenance",
         "unavailable": "Unavailable",
+        "messageAvailable": "System is running normally.",
+        "messageMaintenance": "System is undergoing scheduled maintenance.",
+        "messageUnavailable": "System is currently unavailable.",
         "errorTitle": "Could not load status",
         "staleLabel": "Last known status",
         "retry": "Retry"