feat(display): systemd watchdog and heartbeat for a frozen render loop (#687)

If the render loop gets stuck inside a plugin's display(), ledmatrix.service
stays active and the panel stays frozen. This adds a way to detect that.

- src/display_watchdog.py (standard library only) sends sd_notify over
  $NOTIFY_SOCKET and writes /run/ledmatrix/display-heartbeat.json. Only the
  render thread counts: beats from other threads are ignored.
- ledmatrix.service: WatchdogSec=120, NotifyAccess=main,
  RuntimeDirectory=ledmatrix (0755), RestartSteps=4 and
  RestartMaxDelaySec=2min. It stays Type=simple. run.py widens the watchdog
  to 15 min for start-up, and load_plugin() does the same on the render
  thread. The loop arms after its first frame.
- /api/v3/health adds checks.display_loop: running, stalled (no heartbeat
  for over 60s, which makes the status degraded) or not_reported. With web
  login on, a caller who is not logged in still gets only healthy/degraded,
  and a stall degrades that answer.
- The update verifier requires a fresh heartbeat from the restarted display
  when the display it replaced was writing one. A frozen panel is rolled
  back.
- Existing installs get the systemd watchdog only after install_service.sh
  is re-run. The heartbeat works right away.

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

docs/REST_API_REFERENCE.md

@@ -2101,9 +2101,17 @@ Health of the web interface, display service, config file, plugin system and
 display snapshot. `data.status` is `healthy` or `degraded`, with
 `data.services` and `data.checks`.
 
+`data.checks.display_loop` is the display's render-loop heartbeat: `running`
+(with `heartbeat_age_seconds`), `stalled` (no heartbeat for 60s: the panel is
+frozen even if the service is active; the status turns `degraded`), or
+`not_reported` when the display writes none (not started yet, the dev server,
+Windows), which does not affect the status.
+
 Open even when the web login is on, for uptime monitors; a caller that is not
 logged in (and has no token) then gets only `{"status": "success", "data":
-{"status": "healthy" | "degraded"}}`.
+{"status": "healthy" | "degraded"}}`. A stalled render loop still shows there
+as `degraded`; the `checks` detail is only for logged-in callers, tokens and
+requests from the Pi itself.
 
 ### Hardware Status