The iOS app's icon count, pushed by ipx (#433)

Background refresh runs when iOS chooses, and for this app that has
been rarely, so the icon's unread count lagged until the app was
opened (#421). Now ipx sends the count:
- ipx: a push_devices table and POST/DELETE /api/push/devices. With
  [push] set to an APNs key, a task compares each phone's All
  Subscriptions unread count with the last sent, once a minute, and
  sends a changed one to Apple as a badge-only push: no alert, no
  sound. A token Apple says is gone is forgotten. The signing token is
  ES256 over the team and time, made again every forty minutes, inside
  Apple's limits.
- iOS: the aps-environment entitlement. The app registers for push
  once badge permission is given, each launch, and hands its token to
  the page.
- Page: registers the token with ipx as the person signed in, since
  the player's address (ipxapi) lets through only the player's paths,
  and removes it on signing out.

jsonwebtoken here is built without PEM support, so the .p8 key's
base64 is decoded in push.rs rather than turning on a feature and
changing Cargo.lock for one file.

cargo test -p ipx on macOS: 182 pass; the 3 that fail read /proc and
fail there without this change too.

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

ios/CHANGELOG.md

@@ -12,4 +12,5 @@ The iOS app, `ios/`. Format: [Keep a Changelog](https://keepachangelog.com/en/1.
 - Episodes kept on the phone, with the page's Keep button, play with no connection: the app fetches the file in the background, even with the app closed, and plays it from the phone; where you got to and what you finished offline is sent once the connection is back. Signing out removes them (#425).
 - A sleep timer in the player, kept by the phone, so it pauses on time with the screen locked, fading out over the last ten seconds; and an AirPlay button beside it, for another speaker or a TV (#429).
 - iPX opens with no connection, showing the lists it last had, as it does in a browser: the page's service worker now runs in the app. This is for `ipodderx.sdf1.net` only, and links to other sites open in Safari (#435).
+- The icon's count stays right with the app closed: ipx sends it to the phone when it changes, once its `[push]` is set up, in place of waiting for iOS's background refresh (#433).
 - Presses are felt: a light tap on play, pause, flag and pull to refresh, a success on marking read, a warning with an error (#430).