Exchange a frontend pairing code

Exchanges a short-lived, single-use code created by `oore frontend invite` for the backend trusted-proxy proof and configured identity header. This endpoint accepts requests only from configured trusted-proxy peer CIDRs.

curl -X POST "https://example.com/v1/frontend/pair" \  -H "Content-Type: application/json" \  -d '{    "code": "string"  }'

{  "backend_proof": "string",  "user_email_header": "string"}