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>

smoothschedule/config/settings/multitenancy.py

@@ -14,7 +14,8 @@ from .base import INSTALLED_APPS, MIDDLEWARE, DATABASES, LOGGING, env
 SHARED_APPS = [
     'django_tenants',  # Must be first
     'core',  # Core models (Tenant, Domain, PermissionGrant)
-    
+    'platform_admin.apps.PlatformAdminConfig',  # Platform management (TenantInvitation, etc.)
+
     # Django built-ins (must be in shared
     'django.contrib.contenttypes',
     'django.contrib.auth',
@@ -23,10 +24,10 @@ SHARED_APPS = [
     'django.contrib.messages',
     'django.contrib.staticfiles',
     'django.contrib.admin',
-    
+
     # Users app (shared across tenants)
     'smoothschedule.users',
-    
+
     # Third-party apps that should be shared
     'rest_framework',
     'rest_framework.authtoken',