[routes-b] GET /api/routes-b/exchange-rate - USDC to NGN exchange rate

[davedumto]

Summary

Implement the exchange-rate endpoint under /api/routes-b. USDC to NGN exchange rate.

Requirements

• Create handler at app/api/routes-b/exchange-rate/route.ts
• Implement methods: GET
• Add request validation, auth and ownership checks, and structured error handling
• Add unit tests for the happy path and key failure modes

Acceptance criteria

• Handler implemented for each listed method at the documented path
• Validation and error envelopes match existing routes conventions
• Unit tests pass in CI