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/App.tsx

@@ -63,6 +63,7 @@ const PlatformEmailAddresses = React.lazy(() => import('./pages/platform/Platfor
 const PlatformUsers = React.lazy(() => import('./pages/platform/PlatformUsers'));
 const PlatformStaff = React.lazy(() => import('./pages/platform/PlatformStaff'));
 const PlatformSettings = React.lazy(() => import('./pages/platform/PlatformSettings'));
+const BillingManagement = React.lazy(() => import('./pages/platform/BillingManagement'));
 const ProfileSettings = React.lazy(() => import('./pages/ProfileSettings'));
 const VerifyEmail = React.lazy(() => import('./pages/VerifyEmail'));
 const EmailVerificationRequired = React.lazy(() => import('./pages/EmailVerificationRequired'));
@@ -495,7 +496,10 @@ const AppContent: React.FC = () => {
             <Route path="/help/plugins" element={<HelpPluginDocs />} />
             <Route path="/help/email" element={<HelpEmailSettings />} />
             {user.role === 'superuser' && (
-              <Route path="/platform/settings" element={<PlatformSettings />} />
+              <>
+                <Route path="/platform/settings" element={<PlatformSettings />} />
+                <Route path="/platform/billing" element={<BillingManagement />} />
+              </>
             )}
             <Route path="/platform/profile" element={<ProfileSettings />} />
             <Route path="/verify-email" element={<VerifyEmail />} />