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/SubscriptionPlanConfig.cs

@@ -46,6 +46,9 @@ public class SubscriptionPlanConfig : BaseEntity
     /// <summary>When true, companies on this plan can use the AI Inventory Assist lookup feature.</summary>
     public bool AllowAiInventoryAssist { get; set; } = false;
 
+    /// <summary>When true, companies on this plan can run the AI Catalog Price Check (Enterprise only).</summary>
+    public bool AllowAiCatalogPriceCheck { get; set; } = false;
+
     public bool IsActive { get; set; } = true;
     public int SortOrder { get; set; }
 }