Support multiple named secrets in [secrets] config section. During
FakeTLS handshake each secret is tried until HMAC validates. Matched
secret name is logged and used for per-user statistics tracking.

Built-in HTTP stats API (configurable via api-bind-to) exposes
GET /stats with per-user connection counts, bytes in/out, and
last-seen timestamps.

Backward compatible: single "secret" config key still works.

On some servers ifconfig.co is unreachable (e.g. Hetzner, AdGuard DNS
blocklists), causing 'mtg doctor' SNI-DNS check and 'mtg access' link
generation to fail. New config options allow specifying public IPs
manually, with automatic detection as fallback.

Fixes #405