feat(backend): cross-border payments with FX conversion (#920)

[sandrawillow001-afk]

Summary

Implements cross-border payments end to end: a corridor-aware quote holds an auditable FX rate and converts the amount, and a payment consumes that quote before the settlement rail confirms or rejects it. Composes with the existing FX service (#626).

Closes #920

Changes

• New CrossBorderPaymentService (backend/src/services/cross-border/):
  • corridor registry for USD/EUR/GBP/XLM with limits, settlement rails (SEPA, ACH, Faster Payments, Stellar), fees, and settlement estimates
  • createQuote in source or target amount mode with a transparent fee breakdown, corridor limits, and a rate-hold expiry
  • initiatePayment with idempotency keys, plus completePayment / failPayment status transitions
• New routes under /api/v1/cross-border: corridors, quote, quotes/:id, payments, payments/:id, payments/:id/complete, payments/:id/fail.
• Mounted the new router and the previously unwired FX router at /api/v1/fx.
• Docs: backend/docs/CROSS_BORDER_PAYMENTS.md.

Pricing model

fxFee        = sourceAmount × corridor.fxFeePct
fees.total   = fxFee + corridor.fixedFee
convertible  = sourceAmount − fees.total
targetAmount = convertible × fxRate

Testing

• cd backend && npx vitest run src/services/__tests__/cross-border-service.test.ts → 16 passed
• tsc --noEmit and eslint clean for all new/changed files

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