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>

systemd/README.md

@@ -8,6 +8,10 @@ This directory contains systemd service unit files for LEDMatrix services.
   - Runs the display controller (`run.py`)
   - Starts automatically on boot
   - Runs as root for hardware access
+  - Restarted by systemd's watchdog (`WatchdogSec=120`) when its render loop
+    stops checking in, e.g. stuck inside a plugin; the loop writes a heartbeat
+    to `/run/ledmatrix/display-heartbeat.json` (`RuntimeDirectory=`) that the
+    web interface's health check reads. See `src/display_watchdog.py`
 
 - **`ledmatrix-web.service`** - Web interface service
   - Runs the web interface conditionally based on config