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>

src/vegas_mode/stream_manager.py

@@ -21,6 +21,7 @@ from collections import deque
 from dataclasses import dataclass, field
 from PIL import Image
 
+from src import display_watchdog
 from src.vegas_mode.config import VegasModeConfig
 from src.vegas_mode.plugin_adapter import PluginAdapter
 from src.plugin_system.base_plugin import VegasDisplayMode, resolve_vegas_participation
@@ -562,6 +563,9 @@ class StreamManager:
         Returns:
             ContentSegment or None if fetch failed
         """
+        # Composing a cycle fetches plugin after plugin on the render thread
+        # (on the prefetch thread this is ignored), so check in between.
+        display_watchdog.beat()
         try:
             if not hasattr(self.plugin_manager, 'plugins'):
                 logger.warning("[%s] plugin_manager has no plugins attribute", plugin_id)