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/useBusiness.ts

@@ -60,12 +60,14 @@ export const useCurrentBusiness = () => {
           white_label: false,
           custom_oauth: false,
           plugins: false,
+          tasks: false,
           export_data: false,
           video_conferencing: false,
           two_factor_auth: false,
           masked_calling: false,
           pos_system: false,
           mobile_app: false,
+          contracts: false,
         },
       };
     },