docs/API.md is out of sync with actual routes in src/routes/mod.rs

[abayomicornelius]

docs/API.md documents endpoints such as POST /send, POST /payments/preview, GET /transactions/:address, GET /contacts, and GET /rates/xlm, none of which match the real routes registered in src/routes/mod.rs (POST /api/payments/send, GET /api/transactions/:id, GET /api/rates?from=&to=).

A new contributor or API consumer following docs/API.md will hit 404s on every endpoint. Rewrite docs/API.md to match the real router, or delete it in favor of the more accurate README API Reference section.