Add TenantCustomTier system and fix BusinessEditModal feature loading

Backend:
- Add TenantCustomTier model for per-tenant feature overrides
- Update EntitlementService to check custom tier before plan features
- Add custom_tier action on TenantViewSet (GET/PUT/DELETE)
- Add Celery task for grace period management (30-day expiry)

Frontend:
- Add DynamicFeaturesEditor component for dynamic feature management
- Fix BusinessEditModal to load features from plan defaults when no custom tier
- Update limits (max_users, max_resources, etc.) to use featureValues
- Remove outdated canonical feature check from FeaturePicker (removes warning icons)
- Add useBillingPlans hook for accessing billing system data
- Add custom tier API functions to platform.ts

Features now follow consistent rules:
- Load from plan defaults when no custom tier exists
- Load from custom tier when one exists
- Reset to plan defaults when plan changes
- Save to custom tier on edit

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

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

frontend/src/hooks/useBusiness.ts

@@ -38,7 +38,7 @@ export const useCurrentBusiness = () => {
         timezone: data.timezone || 'America/New_York',
         timezoneDisplayMode: data.timezone_display_mode || 'business',
         whitelabelEnabled: data.whitelabel_enabled,
-        plan: data.tier, // Map tier to plan
+        plan: data.plan,
         status: data.status,
         joinedAt: data.created_at ? new Date(data.created_at) : undefined,
         resourcesCanReschedule: data.resources_can_reschedule,
@@ -72,6 +72,7 @@ export const useCurrentBusiness = () => {
           pos_system: false,
           mobile_app: false,
           contracts: false,
+          multi_location: false,
         },
       };
     },