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- Remove outdated canonical feature check from FeaturePicker (removes warning icons)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/usePlanFeatures.ts

@@ -93,6 +93,7 @@ export const FEATURE_NAMES: Record<FeatureKey, string> = {
   pos_system: 'POS System',
   mobile_app: 'Mobile App',
   contracts: 'Contracts',
+  multi_location: 'Multiple Locations',
 };
 
 /**
@@ -115,4 +116,5 @@ export const FEATURE_DESCRIPTIONS: Record<FeatureKey, string> = {
   pos_system: 'Process in-person payments with Point of Sale',
   mobile_app: 'Access SmoothSchedule on mobile devices',
   contracts: 'Create and manage contracts with customers',
+  multi_location: 'Manage multiple business locations with separate resources and services',
 };