Prometheus metrics for the daemon's state between requests (#194)

Logs and traces show requests, scans and downloads; nothing showed what
the daemon holds meanwhile. /metrics, behind sign-in, gives database
connections in use and idle against the pool's maximum, commands waiting
for the worker, live-update listeners, free torrent slots, episodes
playing through DeadAir, the library's counts, the tokio runtime's
workers, tasks and queue, and the process's memory and open files, in
Prometheus's text format, written by hand rather than with a client
library. Not logged or traced: Prometheus asks every 15 s. The iPX
dashboard gains a Daemon row of them.

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

CHANGELOG.md

@@ -9,6 +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).
 - 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).