[Docs] Update docs/api-reference.md — add Payments endpoints and correct stale contract routes

[gregemax]

Summary

docs/api-reference.md was written before the Payments module was added (Aug 2026). It also documents some contract routes that no longer match their implementation.

What's missing

New section: Payments

GET  /payments/contracts/:contractId  — list payments for a contract
GET  /payments/tx/:txHash            — look up a payment by Stellar tx hash
GET  /payments/earnings              — freelancer earnings aggregate (issue #88, pending)
POST /payments/withdraw              — SEP-24 off-ramp (issue #89, pending)

For each endpoint document: auth requirement, path/query params, example request, example response, error codes.

Updates to existing sections

• Verify all contract route signatures still match contracts.controller.ts
• Add the GET /users admin list endpoint (added 2026-08-01)
• Add the version and get_admin contract entrypoints to the Smart Contract section

Acceptance criteria

• Payments section added
• All documented routes verified against source code
• Example responses use realistic Stellar values (real public key formats, XLM amounts)

Effort estimate

~2–3 hours. Good first issue for someone learning the codebase.

Related

• docs/api-reference.md — file to update
• src/payments/payments.controller.ts