feat(contracts): Add contracts permission to subscription tiers

- Add contracts_enabled field to SubscriptionPlan model
- Add contracts toggle to plan create/edit modal in platform settings
- Hide contracts menu item for tenants without contracts permission
- Protect /contracts routes with canUse('contracts') check
- Add HasContractsPermission to contracts API ViewSets
- Add contracts to PlanPermissions interface and feature definitions

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/hooks/usePlatformSettings.ts

@@ -51,6 +51,8 @@ export interface SubscriptionPlan {
   masked_calling_price_per_minute_cents: number;
   proxy_number_enabled: boolean;
   proxy_number_monthly_fee_cents: number;
+  // Contracts feature
+  contracts_enabled: boolean;
   // Default credit settings
   default_auto_reload_enabled: boolean;
   default_auto_reload_threshold_cents: number;
@@ -82,6 +84,8 @@ export interface SubscriptionPlanCreate {
   masked_calling_price_per_minute_cents?: number;
   proxy_number_enabled?: boolean;
   proxy_number_monthly_fee_cents?: number;
+  // Contracts feature
+  contracts_enabled?: boolean;
   // Default credit settings
   default_auto_reload_enabled?: boolean;
   default_auto_reload_threshold_cents?: number;