Refactor billing system: add-ons in wizard, remove business_tier, move to top-level app

- Add add-ons step to plan creation wizard (step 4 of 5)
- Remove redundant business_tier field from both billing systems:
  - commerce.billing.PlanVersion (new system)
  - platform.admin.SubscriptionPlan (legacy system)
- Move billing app from commerce.billing to top-level smoothschedule.billing
- Create BillingManagement page at /platform/billing with sidebar link
- Update plan matching logic to use plan.name instead of business_tier

Frontend:
- Add BillingManagement.tsx page
- Add BillingPlansTab.tsx with unified plan wizard
- Add useBillingAdmin.ts hooks
- Update TenantInviteModal, BusinessEditModal, BillingSettings to use plan.name
- Remove business_tier from usePlatformSettings, payments.ts types

Backend:
- Move billing app to smoothschedule/billing/
- Add migrations 0006-0009 for plan version settings, feature seeding, business_tier removal
- Add platform_admin migration 0013 to remove business_tier
- Update seed_subscription_plans command
- Update tasks.py to map tier by plan name

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/components/PlatformSidebar.tsx

@@ -1,7 +1,7 @@
 import React from 'react';
 import { useTranslation } from 'react-i18next';
 import { Link, useLocation } from 'react-router-dom';
-import { LayoutDashboard, Building2, MessageSquare, Settings, Users, Shield, HelpCircle, Code, Mail } from 'lucide-react';
+import { LayoutDashboard, Building2, MessageSquare, Settings, Users, Shield, HelpCircle, Code, Mail, CreditCard } from 'lucide-react';
 import { User } from '../types';
 import SmoothScheduleLogo from './SmoothScheduleLogo';
 
@@ -75,6 +75,10 @@ const PlatformSidebar: React.FC<PlatformSidebarProps> = ({ user, isCollapsed, to
               <Shield size={18} className="shrink-0" />
               {!isCollapsed && <span>{t('nav.staff')}</span>}
             </Link>
+            <Link to="/platform/billing" className={getNavClass('/platform/billing')} title="Billing Management">
+              <CreditCard size={18} className="shrink-0" />
+              {!isCollapsed && <span>Billing</span>}
+            </Link>
             <Link to="/platform/settings" className={getNavClass('/platform/settings')} title={t('nav.platformSettings')}>
               <Settings size={18} className="shrink-0" />
               {!isCollapsed && <span>{t('nav.platformSettings')}</span>}