Add three-layer feature gating for AI Catalog Price Check

Adds platform-level, plan-level (Enterprise only), and per-company
toggles for the AI Catalog Price Check feature. Includes:
- Company.AiCatalogPriceCheckEnabled per-company flag
- SubscriptionPlanConfig.AllowAiCatalogPriceCheck plan-level flag
- PlatformSetting 'AiCatalogPriceCheckEnabled' global kill switch
- IPlatformSettingsService.GetBoolAsync helper
- ISubscriptionService.CanUseAiCatalogPriceCheckAsync
- UI controls in Companies/Edit, PlatformSubscription/Edit+Index,
  and SubscriptionManagement/Manage
- Migration AddAiCatalogPriceCheckGating applied

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

src/PowderCoating.Web/Views/CatalogItems/AiPriceCheck.cshtml

@@ -94,7 +94,16 @@
     <a asp-action="Index" class="btn btn-outline-secondary btn-sm">
         <i class="bi bi-arrow-left me-1"></i> Back to Catalog
     </a>
-    @if (ViewBag.NextRunAvailable != null)
+    @if (!(bool)(ViewBag.AiPriceCheckEnabled ?? true))
+    {
+        <div class="text-end">
+            <button class="btn btn-primary" disabled>
+                <i class="bi bi-robot me-2"></i>Analyze Catalog with AI
+            </button>
+            <div class="small text-muted mt-1">Available on the Enterprise plan</div>
+        </div>
+    }
+    else if (ViewBag.NextRunAvailable != null)
     {
         <div class="text-end">
             <button class="btn btn-primary" disabled>