fix(service): survive corrupt health cache and clean exits

Three independent failure modes that each end with a dark panel and no
automatic recovery.

1. PluginHealthTracker._load_health_state returned the cached value
   verbatim. If that value is not a dict, every caller raises
   AttributeError: 'list' object has no attribute 'get' — during
   DisplayController.__init__, so the process dies before the display
   loop starts. systemd restarts it, the same bad entry is read back
   from disk, and it dies again: an unattended restart loop that
   survives reboots because the cause is persisted. Observed in the
   field with plugin_health:<id> holding an unrelated plugin's list
   payload. Now non-dict entries are discarded with a warning and the
   defaults are rebuilt.

2. ledmatrix.service used Restart=on-failure, so any exit with status 0
   left the unit stopped and the panel dark indefinitely — systemd
   treats it as success and never brings it back. Restart=always.

3. ledmatrix-wifi-monitor.service used StandardOutput=syslog, which
   systemd has marked obsolete; it warns and rewrites it to journal on
   every load.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

systemd/ledmatrix.service

@@ -9,7 +9,11 @@ User=root
 WorkingDirectory=__PROJECT_ROOT_DIR__
 Environment=PYTHONDONTWRITEBYTECODE=1
 ExecStart=/usr/bin/python3 __PROJECT_ROOT_DIR__/run.py
-Restart=on-failure
+# Restart=always, not on-failure: run.py exiting 0 (a clean shutdown path taken
+# for a reason that no longer applies, e.g. a config reload) would otherwise leave
+# the service stopped and the panel dark indefinitely, with systemd considering
+# that a successful outcome and never bringing it back.
+Restart=always
 RestartSec=10
 StandardOutput=journal
 StandardError=journal