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>

src/SlpModularCms.Modules.Availability/Services/PersistentAvailabilityService.cs

@@ -54,6 +54,24 @@ public class PersistentAvailabilityService : IAvailabilityService
         }
     }
 
+    public async Task<AvailabilityStatusDetails> GetStatusDetailsAsync()
+    {
+        try
+        {
+            var state = await _context.AvailabilityStates.FirstOrDefaultAsync();
+            if (state == null)
+                return new AvailabilityStatusDetails(AvailabilityStatus.Available, null);
+
+            return new AvailabilityStatusDetails(state.Status, state.Message);
+        }
+        catch (Exception)
+        {
+            _lastErrorTime = DateTimeOffset.UtcNow;
+            _cachedStatus = AvailabilityStatus.Available;
+            return new AvailabilityStatusDetails(_cachedStatus, null);
+        }
+    }
+
     /// <summary>
     /// Update de globale systeemstatus.
     /// </summary>