Add patient payment history and facility ledger APIs

[victor-134]

Summary

Add GET /payments/mine and GET /payments/facility/ledger with role enforcement, strict status/date filters, stable cursor pagination, and facility-scoped ledger totals.

Payment rows include on-chain explorer references and return BigInt values as decimal strings. Facility totals are grouped in PostgreSQL, not summed in JavaScript. The current custodial v1 schema and contract path do not support refund_after, so refundAfter is explicitly null; native XLM is identified as the asset. API docs and the Phase 5.5 roadmap are updated.

Tests

• npm run typecheck
• npm run test (93 passed)
• npm run test:integration (160 passed, including 3 new payment-history tests)
• npm run audit:prod (passed under the repository's documented Axios advisory policy)
• npm run build

Closes #90

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