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/Companies/Edit.cshtml

@@ -162,6 +162,13 @@
                                 </div>
                                 <div class="form-text">Allow this company to use AI lookup on inventory items.</div>
                             </div>
+                            <div class="col-md-4">
+                                <div class="form-check form-switch">
+                                    <input asp-for="AiCatalogPriceCheckEnabled" class="form-check-input" type="checkbox" />
+                                    <label asp-for="AiCatalogPriceCheckEnabled" class="form-check-label fw-medium">AI Catalog Price Check</label>
+                                </div>
+                                <div class="form-text">Allow this company to run AI-powered catalog price analysis.</div>
+                            </div>
                             <div class="col-md-4">
                                 <label asp-for="MaxAiPhotoQuotesPerMonthOverride" class="form-label">Monthly AI Quote Limit Override</label>
                                 <input asp-for="MaxAiPhotoQuotesPerMonthOverride" type="number" class="form-control" min="-1" placeholder="Leave blank to use plan default" />