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/SubscriptionManagement/Manage.cshtml

@@ -293,6 +293,15 @@
                         </div>
                         <div class="form-text">Allow AI-powered inventory lookups.</div>
                     </div>
+                    <div class="col-md-4">
+                        <div class="form-check form-switch">
+                            <input class="form-check-input" type="checkbox" role="switch"
+                                   name="aiCatalogPriceCheckEnabled" value="true" id="aiCatalogPriceCheck"
+                                   @(Model.AiCatalogPriceCheckEnabled ? "checked" : "") />
+                            <label class="form-check-label" for="aiCatalogPriceCheck">AI Catalog Price Check</label>
+                        </div>
+                        <div class="form-text">Allow AI-powered catalog price analysis.</div>
+                    </div>
                     <div class="col-md-4">
                         <label class="form-label small fw-medium">AI Photo Quotes / Month Override</label>
                         <input type="number" name="maxAiPhotoQuotesPerMonthOverride" class="form-control form-control-sm"