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>

CHANGELOG.md

@@ -9,7 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
-- `/metrics` gives Prometheus what the daemon holds between requests: database connections in use against the pool's size, commands waiting for the worker, live-update listeners, the runtime's tasks, memory and open files. The iPX dashboard graphs them in a Daemon row (#194).
+- With `[web] metrics_bind` set, `/metrics` gives Prometheus, on a thread and port of its own, what the daemon holds between requests: database connections in use against the pool's size, commands waiting for the worker, live-update listeners, the runtime's tasks, memory and open files. The iPX dashboard graphs them in a Daemon row (#194).
 - The page reports its errors, the player's, how long play takes to start and how long it stalls to the server's log, and the requests one tap sets off share a trace, the playlist, audio and saved place of what it played included (#190, #192).
 - Saving an episode from a show with DeadAir on saves it without its dead air, at your breathing room (#186).
 - Choose the pause DeadAir leaves where it cuts a silence, from 0.2 to 0.5 seconds, for each show in its settings, as its breathing room, on a slider from brisk to relaxed. It is 0.3 seconds unless you change it, up from 0.2, which made speech sound rushed (#182, #183).