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.Application/DTOs/Company/CompanyDtos.cs

@@ -158,6 +158,7 @@ public class UpdateCompanyDto
     // AI feature flags
     public bool AiPhotoQuotesEnabled { get; set; }
     public bool AiInventoryAssistEnabled { get; set; }
+    public bool AiCatalogPriceCheckEnabled { get; set; }
     public int? MaxAiPhotoQuotesPerMonthOverride { get; set; }
 
     // Per-company feature overrides (null = use plan default)

src/PowderCoating.Application/DTOs/Subscription/SubscriptionPlanConfigDto.cs

@@ -24,6 +24,7 @@ public class SubscriptionPlanConfigDto
     public bool AllowAccounting { get; set; }
     public bool AllowAiPhotoQuotes { get; set; }
     public bool AllowAiInventoryAssist { get; set; }
+    public bool AllowAiCatalogPriceCheck { get; set; }
     public bool IsActive { get; set; }
     public int SortOrder { get; set; }
 }
@@ -70,6 +71,7 @@ public class UpdateSubscriptionPlanConfigDto
     public bool AllowAccounting { get; set; }
     public bool AllowAiPhotoQuotes { get; set; }
     public bool AllowAiInventoryAssist { get; set; }
+    public bool AllowAiCatalogPriceCheck { get; set; }
 
     public bool IsActive { get; set; }
 }