feat: Implement tenant invitation system with onboarding wizard

Backend Implementation:
- Add TenantInvitation model with lifecycle management (PENDING/ACCEPTED/EXPIRED/CANCELLED)
- Create platform admin API endpoints for invitation CRUD operations
- Add public token-based endpoints for invitation retrieval and acceptance
- Implement schema_context wrappers to ensure tenant operations run in public schema
- Add tenant permissions: can_manage_oauth_credentials, can_accept_payments, can_use_custom_domain, can_white_label, can_api_access
- Fix tenant update/create serializers to handle multi-schema environment
- Add migrations for tenant permissions and invitation system

Frontend Implementation:
- Create TenantInviteModal with comprehensive invitation form (350 lines)
  - Email, business name, subscription tier configuration
  - Custom user/resource limits
  - Platform permissions toggles
  - Future feature flags (video conferencing, event types, calendars, 2FA, logs, data deletion, POS, mobile app)
- Build TenantOnboardPage with 4-step wizard for invitation acceptance
  - Step 1: Account setup (email, password, name)
  - Step 2: Business details (name, subdomain, contact)
  - Step 3: Payment setup (conditional based on permissions)
  - Step 4: Success confirmation with redirect
- Extract BusinessCreateModal and BusinessEditModal into separate components
- Refactor PlatformBusinesses from 1080 lines to 220 lines (80% reduction)
- Add inactive businesses dropdown section (similar to staff page pattern)
- Update masquerade button styling to match Users page
- Remove deprecated "Add New Tenant" functionality in favor of invitation flow
- Add /tenant-onboard route for public access

API Integration:
- Add platform.ts API functions for tenant invitations
- Create React Query hooks in usePlatform.ts for invitation management
- Implement proper error handling and success states
- Add TypeScript interfaces for invitation types

Testing:
- Verified end-to-end invitation flow from creation to acceptance
- Confirmed tenant, domain, and owner user creation
- Validated schema context fixes for multi-tenant environment
- Tested active/inactive business filtering

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

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

frontend/src/hooks/useBusinessOAuthCredentials.ts

@@ -4,15 +4,17 @@
 
 import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query';
 import { getBusinessOAuthCredentials, updateBusinessOAuthCredentials } from '../api/business';
-import { BusinessOAuthCredentials } from '../types';
+import { BusinessOAuthCredentialsResponse } from '../types';
 
 /**
  * Fetch business OAuth credentials
  */
 export const useBusinessOAuthCredentials = () => {
-  return useQuery({
+  return useQuery<BusinessOAuthCredentialsResponse>({
     queryKey: ['businessOAuthCredentials'],
     queryFn: getBusinessOAuthCredentials,
+    retry: false, // Don't retry on 404
+    staleTime: 5 * 60 * 1000, // 5 minutes
   });
 };
 
@@ -23,8 +25,10 @@ export const useUpdateBusinessOAuthCredentials = () => {
   const queryClient = useQueryClient();
 
   return useMutation({
-    mutationFn: (credentials: Partial<BusinessOAuthCredentials>) =>
-      updateBusinessOAuthCredentials(credentials),
+    mutationFn: (data: {
+      credentials?: Record<string, { client_id?: string; client_secret?: string }>;
+      useCustomCredentials?: boolean;
+    }) => updateBusinessOAuthCredentials(data),
     onSuccess: (data) => {
       queryClient.setQueryData(['businessOAuthCredentials'], data);
     },