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>

frontend/src/types.ts

@@ -209,6 +209,12 @@ export interface Service {
   photos?: string[];
   created_at?: string; // Used for quota overage calculation (oldest archived first)
   is_archived_by_quota?: boolean; // True if archived due to quota overage
+  // Variable pricing fields
+  variable_pricing?: boolean; // If true, final price is determined after service completion
+  deposit_amount?: number | null; // Fixed deposit amount
+  deposit_percent?: number | null; // Deposit as percentage (0-100)
+  requires_saved_payment_method?: boolean; // If true, customer must have saved card
+  deposit_display?: string | null; // Human-readable deposit description
 }
 
 export interface Metric {