Resilience : ecran pause, reconnexion WS auto, watchdog systemd, WiFi monitor

- WebSocket : reconnexion avec backoff exponentiel au lieu de reload page
- Ecran pause avec tasse de cafe quand le backend est injoignable
- Endpoint /api/health pour monitoring externe
- Watchdog systemd (sd_notify READY=1 + WATCHDOG=1 toutes les 10s)
- Service systemd durci (Type=notify, WatchdogSec=30, KillMode, limites)
- WiFi watchdog : script + timer systemd, verifie la gateway toutes les 60s
- Destinations : fallback url_tunnel (WireGuard) en priorite

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

scripts/photobooth.service

@@ -1,15 +1,25 @@
 [Unit]
-Description=Photobooth
+Description=Photobooth backend
 After=network.target graphical.target
+Wants=graphical.target
 
 [Service]
-Type=simple
+Type=notify
 User=jules
-WorkingDirectory=/home/jules/Documents/Projet/photobooth-app
-ExecStart=/home/jules/Documents/Projet/photobooth-app/scripts/start.sh
+WorkingDirectory=/home/jules/photobooth
+ExecStart=/home/jules/photobooth/scripts/start.sh
+ExecStopPost=/bin/bash -c 'pkill -9 -f "python.*backend.main" 2>/dev/null; sleep 1'
 Restart=always
-RestartSec=5
+RestartSec=3
+WatchdogSec=30
+KillMode=control-group
+KillSignal=SIGTERM
+TimeoutStopSec=10
+StartLimitIntervalSec=120
+StartLimitBurst=10
 Environment=DISPLAY=:0
+Environment=XAUTHORITY=/home/jules/.Xauthority
+AmbientCapabilities=CAP_NET_BIND_SERVICE
 
 [Install]
 WantedBy=graphical.target