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/pages/platform/components/TenantInviteModal.tsx

@@ -150,7 +150,7 @@ const TenantInviteModal: React.FC<TenantInviteModalProps> = ({ isOpen, onClose }
         'ENTERPRISE': 'Enterprise',
       };
       const plan = subscriptionPlans.find(p =>
-        p.business_tier === tierNameMap[tier] || p.business_tier === tier
+        p.name === tierNameMap[tier] || p.name === tier
       );
       if (plan) {
         const staticDefaults = TIER_DEFAULTS[tier] || TIER_DEFAULTS.FREE;