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/App.tsx

@@ -54,6 +54,7 @@ import ProfileSettings from './pages/ProfileSettings';
 import VerifyEmail from './pages/VerifyEmail';
 import EmailVerificationRequired from './pages/EmailVerificationRequired';
 import AcceptInvitePage from './pages/AcceptInvitePage';
+import TenantOnboardPage from './pages/TenantOnboardPage';
 
 const queryClient = new QueryClient({
   defaultOptions: {
@@ -212,6 +213,7 @@ const AppContent: React.FC = () => {
         <Route path="/oauth/callback/:provider" element={<OAuthCallback />} />
         <Route path="/verify-email" element={<VerifyEmail />} />
         <Route path="/accept-invite" element={<AcceptInvitePage />} />
+        <Route path="/tenant-onboard" element={<TenantOnboardPage />} />
         <Route path="*" element={<Navigate to="/" replace />} />
       </Routes>
     );
@@ -225,6 +227,7 @@ const AppContent: React.FC = () => {
         <Route path="/oauth/callback/:provider" element={<OAuthCallback />} />
         <Route path="/verify-email" element={<VerifyEmail />} />
         <Route path="/accept-invite" element={<AcceptInvitePage />} />
+        <Route path="/tenant-onboard" element={<TenantOnboardPage />} />
         <Route path="*" element={<Navigate to="/login" replace />} />
       </Routes>
     );