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.Core/Availability/AvailabilityStatusDetails.cs

@@ -0,0 +1,6 @@
+namespace SlpModularCms.Core.Availability;
+
+/// <summary>
+/// Status en optionele admin-melding van de systeembeschikbaarheid.
+/// </summary>
+public record AvailabilityStatusDetails(AvailabilityStatus Status, string? Message);

src/SlpModularCms.Core/Availability/IAvailabilityService.cs

@@ -10,4 +10,9 @@ public interface IAvailabilityService
     /// </summary>
     /// <returns>De huidige <see cref="AvailabilityStatus"/>.</returns>
     Task<AvailabilityStatus> IsAvailableAsync();
+
+    /// <summary>
+    /// Geeft de huidige status inclusief de optionele admin-melding terug.
+    /// </summary>
+    Task<AvailabilityStatusDetails> GetStatusDetailsAsync();
 }

src/SlpModularCms.Modules.Availability/Controllers/AvailabilityController.cs

@@ -20,12 +20,12 @@ public class AvailabilityController : ControllerBase
     [AllowAnonymous]
     public async Task<IActionResult> GetStatus()
     {
-        var status = await _availabilityService.IsAvailableAsync();
+        var details = await _availabilityService.GetStatusDetailsAsync();
         return Ok(new
         {
-            Status = status.ToString(),
+            Status = details.Status.ToString(),
             CheckedAt = DateTimeOffset.UtcNow,
-            Message = string.Empty,
+            Message = details.Message ?? string.Empty,
         });
     }
 

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>