feat: complete phase six backoffice operations

Tiku.Api/Middleware/ExceptionHandlingMiddleware.cs

@@ -7,6 +7,7 @@ using Tiku.Application.Commerce;
 using Tiku.Application.Content;
 using Tiku.Application.Growth;
 using Tiku.Application.Points;
+using Tiku.Application.PlatformAdmin;
 using Tiku.Application.QuestionBanks;
 using Tiku.Application.Storage;
 using Tiku.Infrastructure.Content;
@@ -235,6 +236,16 @@ public sealed class ExceptionHandlingMiddleware(
                 return;
             }
 
+            if (exception is PlatformAdminException platformAdminException)
+            {
+                await WriteProblemAsync(
+                    context,
+                    platformAdminException.Message,
+                    PlatformAdminStatusCode(platformAdminException.Code),
+                    platformAdminException.Code);
+                return;
+            }
+
             if (exception is CommerceException commerceException)
             {
                 await WriteProblemAsync(
@@ -469,6 +480,17 @@ public sealed class ExceptionHandlingMiddleware(
         };
     }
 
+    private static int PlatformAdminStatusCode(string code)
+    {
+        return code switch
+        {
+            "platform_access_denied" => StatusCodes.Status403Forbidden,
+            "tenant_slug_exists" => StatusCodes.Status409Conflict,
+            _ when code.EndsWith("_not_found", StringComparison.Ordinal) => StatusCodes.Status404NotFound,
+            _ => StatusCodes.Status400BadRequest
+        };
+    }
+
     private static int PointStatusCode(string code)
     {
         return code switch