Expose Prometheus metrics on a separate port

Metrics are served on METRICS_PORT (default 2112, "off" to disable), never on
the public port. Besides the Go runtime and process metrics this adds:

- http_requests_total and http_request_duration_seconds per route pattern,
  from an Echo middleware.
- musicserver_songs_requested_total and musicserver_songs_played_total by
  pick mode (random, low, classic). The mode is now stored with each queued
  song (migration 000006, not released yet).
- Library size, sync duration, last successful sync, broken songs from the
  last full sync, and the database connection pool.

Refs #35, #36, #37

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Dockerfile

@@ -30,6 +30,8 @@ COPY ./songs/ ./songs/
 # The music volume must be readable by uid 1000, and writable for the .id file cleanup during sync.
 USER 1000:1000
 EXPOSE 8080
+# Prometheus metrics (METRICS_PORT). Keep it off the public proxy.
+EXPOSE 2112
 VOLUME /sorted
 VOLUME /characters