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- Remove outdated canonical feature check from FeaturePicker (removes warning icons)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

@@ -11,6 +11,7 @@ import {
   updateBusiness,
   createBusiness,
   deleteBusiness,
+  changeBusinessPlan,
   PlatformBusinessUpdate,
   PlatformBusinessCreate,
   getTenantInvitations,
@@ -73,6 +74,22 @@ export const useUpdateBusiness = () => {
   });
 };
 
+/**
+ * Hook to change a business's subscription plan (platform admin only)
+ */
+export const useChangeBusinessPlan = () => {
+  const queryClient = useQueryClient();
+
+  return useMutation({
+    mutationFn: ({ businessId, planCode }: { businessId: number; planCode: string }) =>
+      changeBusinessPlan(businessId, planCode),
+    onSuccess: () => {
+      // Invalidate and refetch businesses list
+      queryClient.invalidateQueries({ queryKey: ['platform', 'businesses'] });
+    },
+  });
+};
+
 /**
  * Hook to create a new business (platform admin only)
  */