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/usePlanFeatures.ts

@@ -91,6 +91,7 @@ export const FEATURE_NAMES: Record<FeatureKey, string> = {
   masked_calling: 'Masked Calling',
   pos_system: 'POS System',
   mobile_app: 'Mobile App',
+  contracts: 'Contracts',
 };
 
 /**
@@ -111,4 +112,5 @@ export const FEATURE_DESCRIPTIONS: Record<FeatureKey, string> = {
   masked_calling: 'Use masked phone numbers to protect privacy',
   pos_system: 'Process in-person payments with Point of Sale',
   mobile_app: 'Access SmoothSchedule on mobile devices',
+  contracts: 'Create and manage contracts with customers',
 };