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>

frontend/js/app.js

@@ -118,9 +118,34 @@ function allerA(ecran) {
     chargerGalerie();
   } else if (ecran === 'admin') {
     chargerAdmin();
+    demarrerTimeoutAdmin();
+  } else {
+    arreterTimeoutAdmin();
   }
 }
 
+let _adminTimeoutId = null;
+const ADMIN_TIMEOUT_MS = 30000;
+
+function demarrerTimeoutAdmin() {
+  arreterTimeoutAdmin();
+  _adminTimeoutId = setTimeout(() => { allerA('accueil'); }, ADMIN_TIMEOUT_MS);
+  const ecranAdmin = document.getElementById('ecran-admin');
+  ecranAdmin.removeEventListener('pointerdown', _resetAdminTimeout);
+  ecranAdmin.addEventListener('pointerdown', _resetAdminTimeout);
+}
+
+function _resetAdminTimeout() {
+  if (_adminTimeoutId) {
+    clearTimeout(_adminTimeoutId);
+    _adminTimeoutId = setTimeout(() => { allerA('accueil'); }, ADMIN_TIMEOUT_MS);
+  }
+}
+
+function arreterTimeoutAdmin() {
+  if (_adminTimeoutId) { clearTimeout(_adminTimeoutId); _adminTimeoutId = null; }
+}
+
 function recommencer() {
   photosSession = [];
   photoFinale = null;