PR 2: normalize DiagnosticsController auth; add Maintenance back-links

- DiagnosticsController: replaced raw [Authorize(Roles = "SuperAdmin,Administrator")] with [Authorize(Policy = SuperAdminOnly)] to match every other platform-admin controller; added PowderCoating.Shared.Constants using directive
- DataPurge, DataExport, StorageMigration, SeedData: added "← Maintenance" breadcrumb link at the top of each page so operators know they are in the guarded maintenance area and can navigate back to the hub

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/PowderCoating.Web/Views/DataExport/Index.cshtml

@@ -29,6 +29,11 @@
 }
 
 <div class="container-fluid py-3">
+    <div class="mb-2">
+        <a asp-controller="PlatformAdmin" asp-action="Maintenance" class="text-muted small text-decoration-none">
+            <i class="bi bi-arrow-left me-1"></i>Maintenance
+        </a>
+    </div>
     <div class="d-flex align-items-center justify-content-between mb-3">
         <div>
             <h4 class="mb-0"><i class="bi bi-file-earmark-arrow-down me-2 text-primary"></i>Data Export</h4>