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.Core/Interfaces/ISubscriptionService.cs

@@ -28,6 +28,8 @@ public interface ISubscriptionService
     Task<bool> CanUseAiPhotoQuoteAsync(int companyId);
     /// <summary>Returns (used this month, monthly max). Max = -1 means unlimited.</summary>
     Task<(int Used, int Max)> GetAiPhotoQuoteUsageAsync(int companyId);
+    /// <summary>Returns true if the AI Catalog Price Check is enabled for this company (plan gate + per-company flag).</summary>
+    Task<bool> CanUseAiCatalogPriceCheckAsync(int companyId);
 
     /// <summary>
     /// Returns days until expiry (negative = days past expiry). Returns null if no end date set.