ipx-play: metrics for Prometheus, a dashboard row and a down alert (#458)

ipx-play sent traces and logged JSON but served no /metrics, so nothing showed whether
shared links were opening or failing. It now serves them on metrics_bind (9189 in
production, off the tunnel's port), with the directory's histogram and /proc reading:
requests by method, route and status (by route, so each shared episode is not a row of its
own, and an unknown address is route=""), episodes shared, memory, open files, uptime and
version.

The iPX dashboard gets an ipx-play row, its warnings join the other containers' log panel,
and alerts.py fires when Prometheus cannot reach it. The prod-check skill and docs/play.md
name it, with the scrape job and the OTEL_RESOURCE_ATTRIBUTES tag the trace panels need.

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

crates/play/CHANGELOG.md

@@ -14,5 +14,6 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - A page for each shared episode, at `/e/<id>`: its show, title, date, length and notes, a player that plays the publisher's own file from the time it was shared at (`?t=754` or `?t=12:34`), buttons to subscribe in Apple Podcasts, Overcast, Pocket Casts or Castro, the feed's address, and a preview where the link is pasted (#447).
 - `POST /v1/shares`, for an ipx server with a token from `ipx-play token <name>`: an episode in, its short link out, the same link however often and from whichever server it is shared. An address that looks like a private or paid feed's is refused (#447).
+- Metrics for Prometheus on a port of its own, `metrics_bind`: requests by route and status and their time, episodes shared, memory (#458).
 
 [unreleased]: https://git.sdf1.net/rays/ipx/commits/main/crates/play