feat(httpapi): add Prometheus metrics endpoint

[0dillon]

Summary

To better monitor the service in production, we need a metrics endpoint (e.g. /metrics) that exposes challenge issuance rates, verification success/failure rates, and TOML resolver latencies.

Acceptance Criteria

• Add a /metrics handler.
• Track GET /auth and POST /auth request counts and latencies.
• Expose the metrics in Prometheus format.

Tech Stack

Go, prometheus/client_golang