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/components/Sidebar.tsx

@@ -63,7 +63,9 @@ const Sidebar: React.FC<SidebarProps> = ({ business, user, isCollapsed, toggleCo
   return (
     <div
       className={`flex flex-col h-full text-white shrink-0 transition-all duration-300 ${isCollapsed ? 'w-20' : 'w-64'}`}
-      style={{ backgroundColor: business.primaryColor }}
+      style={{
+        background: `linear-gradient(to bottom right, ${business.primaryColor}, ${business.secondaryColor || business.primaryColor})`
+      }}
     >
       <button
         onClick={toggleCollapse}