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/PlatformSubscription/Index.cshtml

@@ -156,6 +156,19 @@
                                     }
                                 </td>
                             </tr>
+                            <tr>
+                                <td class="text-muted">AI Price Check</td>
+                                <td>
+                                    @if (plan.AllowAiCatalogPriceCheck)
+                                    {
+                                        <span class="badge bg-success">Enabled</span>
+                                    }
+                                    else
+                                    {
+                                        <span class="badge bg-secondary">Disabled</span>
+                                    }
+                                </td>
+                            </tr>
                             <tr class="table-light">
                                 <td colspan="2" class="fw-semibold small text-uppercase text-muted py-1">Stripe</td>
                             </tr>