feat(payments): Add variable pricing with deposit collection

Services can now have variable pricing where:
- Final price is determined after service completion
- A deposit (fixed amount or percentage) is collected at booking
- Customer's saved payment method is charged for remaining balance

Changes:
- Add variable_pricing, deposit_amount, deposit_percent fields to Service model
- Add service FK and final_price fields to Event model
- Add AWAITING_PAYMENT status to Event
- Add SetFinalPriceView endpoint to charge customer's saved card
- Add EventPricingInfoView endpoint for pricing details
- Update Services page with variable pricing toggle and deposit config
- Show "From $X" and deposit info in customer preview

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

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

smoothschedule/payments/urls.py

@@ -39,6 +39,9 @@ from .views import (
     CustomerSetupIntentView,
     CustomerPaymentMethodDeleteView,
     CustomerPaymentMethodDefaultView,
+    # Variable pricing / final charge
+    SetFinalPriceView,
+    EventPricingInfoView,
 )
 
 urlpatterns = [
@@ -84,4 +87,8 @@ urlpatterns = [
     path('customer/setup-intent/', CustomerSetupIntentView.as_view(), name='customer-setup-intent'),
     path('customer/payment-methods/<str:payment_method_id>/', CustomerPaymentMethodDeleteView.as_view(), name='customer-payment-method-delete'),
     path('customer/payment-methods/<str:payment_method_id>/default/', CustomerPaymentMethodDefaultView.as_view(), name='customer-payment-method-default'),
+
+    # Variable pricing / final charge endpoints
+    path('events/<int:event_id>/final-price/', SetFinalPriceView.as_view(), name='set-final-price'),
+    path('events/<int:event_id>/pricing/', EventPricingInfoView.as_view(), name='event-pricing-info'),
 ]