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>

run.py

@@ -14,6 +14,14 @@ project_dir = os.path.dirname(os.path.abspath(__file__))
 if project_dir not in sys.path:
     sys.path.insert(0, project_dir)
 
+# Under systemd the watchdog clock is already running, and start-up (plugin
+# loads, initial updates) takes far longer than the render loop's limit. Widen
+# it before anything slow is imported; the render loop narrows it again once
+# its first frame is on the panel. A no-op outside systemd. Standard library
+# only -- see src/display_watchdog.py.
+from src import display_watchdog
+display_watchdog.watchdog.begin_startup()
+
 # Parse command-line arguments BEFORE any imports
 parser = argparse.ArgumentParser(description='LEDMatrix Display Controller')
 parser.add_argument('-e', '--emulator', action='store_true',