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>

backend/main.py

@@ -2,6 +2,7 @@ import asyncio
 import base64
 import json
 import logging
+import os
 import threading
 import time
 from pathlib import Path
@@ -273,6 +274,24 @@ def _appliquer_config_camera():
     camera.configurer_flash(flash_integre)
 
 
+async def _watchdog_systemd():
+    """Notifie systemd que le service est vivant + surveille la santé."""
+    try:
+        import socket
+        addr = os.environ.get("NOTIFY_SOCKET")
+        if not addr:
+            return
+        sock = socket.socket(socket.AF_UNIX, socket.SOCK_DGRAM)
+        if addr[0] == "@":
+            addr = "\0" + addr[1:]
+        sock.sendto(b"READY=1", addr)
+        while True:
+            await asyncio.sleep(10)
+            sock.sendto(b"WATCHDOG=1", addr)
+    except Exception:
+        pass
+
+
 @asynccontextmanager
 async def lifespan(app: FastAPI):
     """Demarrage et arret de l'application."""
@@ -287,10 +306,12 @@ async def lifespan(app: FastAPI):
     task_dslr = asyncio.create_task(surveiller_dslr())
     task_push = asyncio.create_task(_pusher_preview())
     task_spool = asyncio.create_task(tache_spool_demarrage())
+    task_watchdog = asyncio.create_task(_watchdog_systemd())
     yield
     task_dslr.cancel()
     task_push.cancel()
     task_spool.cancel()
+    task_watchdog.cancel()
     log.info("Arret du photobooth")
     camera.deconnecter()
 
@@ -311,6 +332,18 @@ async def page_admin():
     return FileResponse(str(RACINE / "frontend" / "index.html"))
 
 
+@app.get("/api/health")
+async def api_health():
+    return {
+        "status": "ok",
+        "camera": camera.mode if camera.connectee else "disconnected",
+        "clients": len(clients_ws),
+        "uptime": int(time.time() - _start_time),
+    }
+
+_start_time = time.time()
+
+
 @app.get("/")
 async def page_principale(request: Request):
     from fastapi.responses import RedirectResponse