feat(payments): add cross-account payment splitting

[Clement-coder]

Summary

Implements issue #1720 — cross-account payment splitting.

Revenue sharing between accounts was manual. This PR makes it automatic: a single payment can now be split across multiple destination accounts by percentage with a single API call.

What's in this PR

Layer	File
DB migration	backend/migrations/005_payment_split_records.sql
Interfaces	backend/services/payment/interfaces.ts — PaymentSplit type + splits? on PaymentRequest
Domain errors	backend/services/payment/errors.ts — SPLIT_VALIDATION_ERROR error code
Domain service	backend/services/payment/domain/PaymentSplitService.ts
HTTP controller	backend/services/payment/controller/paymentSplitController.ts
Router	backend/services/payment/router/paymentSplitRouter.ts
Server wiring	backend/server/createApiServer.ts
Package exports	backend/services/payment/index.ts
Tests	backend/services/payment/__tests__/PaymentSplitService.test.ts — 46 tests, all passing

Acceptance criteria

• ✅ Percentages validated to 100 — validateSplits throws SPLIT_VALIDATION_ERROR (HTTP 422) when splits don't sum to 100 (±0.01 tolerance for floating-point)
• ✅ Each split is recorded — SplitEntryResult per destination in the response; payment_split_records / payment_split_entries tables persist outcomes
• ✅ Tests — 46 passing unit tests covering validation, amount arithmetic, happy path, partial failure, idempotency key derivation, metadata forwarding, and all HTTP response codes

API

POST /api/v1/payments/split
Content-Type: application/json

{
  "amount": 10000,
  "currency": "USD",
  "idempotencyKey": "order_abc_split",
  "splits": [
    { "destinationAccountId": "acct_merchant", "percentage": 80 },
    { "destinationAccountId": "acct_platform", "percentage": 20 }
  ]
}

Returns 200 when all splits succeed, 207 Multi-Status when at least one fails (remaining splits are still attempted).

Closes #1720