MVP 1.1 — Implement API endpoints for latest data

[Boxuan-Matty-Lin]

Goal

Expose backend endpoints the frontend can rely on.

Tasks

• Implement GET /api/rates/latest → proxy OXR latest.json (cache: "no-store", hide OXR_APP_ID).

Acceptance

• /api/rates returns { timestamp, base: "USD", rates: {...} } and proper 5xx on failures.

[Boxuan-Matty-Lin]

First stage: get cleaned latest data: