Metrics on a thread and port of their own, from memory alone (#194)

/metrics shared the web server's threads, and answered after a token
lookup and three counting queries, so a pool run dry, one of the things it
is there to show, would have stalled it, and a slow scrape held a web
thread. Now [web] metrics_bind starts a listener on its own OS thread with
a current-thread runtime, reading only what is in memory: pool counts,
channel capacity, the daemon runtime's metrics through its handle, /proc.
No sign-in, as exporters have none; publish the port to the LAN only. The
feed, pending and downloaded counts stay with the healthcheck's log lines,
which the dashboard already reads.

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

docker-compose.yml

@@ -19,6 +19,7 @@ services:
       - ipx.env   # relative: Arcane resolves it inside its own container
     ports:
       - "8099:8099"          # web UI
+      - "192.168.1.130:9187:9187"  # /metrics for Prometheus, LAN only (ipx#194)
       - "6881:6881/tcp"      # BitTorrent peers
       - "6881:6881/udp"      # DHT
     volumes: