Server-side payment observability via standard :telemetry events.

Mirrors the reference SDK payment-event-hooks surface (challenge.created, payment.failed, payment.success) as Elixir telemetry events at challenge, verification, and receipt boundaries.

Payloads are safe to log — no raw credentials or payment proof appear in metadata; only challenge IDs, method/intent names, charge amounts, and high-level error or receipt outcomes.

Events

• [:mpp, :challenge] — challenge issued (402 response / retry)
• [:mpp, :verify, :start] — credential verification began
• [:mpp, :verify, :ok] — verification succeeded
• [:mpp, :verify, :fail] — verification failed
• [:mpp, :receipt] — receipt produced after successful verification

Measurements on [:mpp, :verify, :ok] and [:mpp, :verify, :fail] include :duration (native monotonic time from start to outcome).