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/Entities/Company.cs

@@ -64,6 +64,8 @@ public class Company : BaseEntity
     public bool AiPhotoQuotesEnabled { get; set; } = true;
     /// <summary>Enables/disables the AI Inventory Assist lookup for this company.</summary>
     public bool AiInventoryAssistEnabled { get; set; } = true;
+    /// <summary>Enables/disables the AI Catalog Price Check for this company.</summary>
+    public bool AiCatalogPriceCheckEnabled { get; set; } = true;
 
     /// <summary>
     /// Stores the billing period the customer selected at registration (or last changed on the Billing page).