Add fake payment API

[francoabaroa]

Summary

• add a fake payment record model to the in-memory database
• add payment lifecycle routes: send, list, get, complete, cancel, and fail
• support retry-safe sends with an optional idempotency key
• add route tests covering lifecycle, filtering, idempotency, cancellation, and failure
• document the fake payment API in the README

Bounty

/claim #1

Verification

• git diff --check
• Added route coverage in tests/routes/payments.test.ts

I did not run the test suite locally because this was prepared in an isolated Codex workspace under a no-local-execution constraint for untrusted bounty repositories.