Add per-camera capture watchdog with stall and recovery notifications

capture-watchdog.sh: long-running script that checks the newest JPEG mtime
in each camera's output directory every CAPTURE_STALE_SECS/3 seconds (min 5s).
Sends a stall notification via notify.sh if no new frame has arrived within
CAPTURE_STALE_SECS. Sends a recovery notification once new frames resume.
Only alerts if the camera was previously working (avoids false positives on
first start or overnight before the first frame of the day).

sky-cam.conf: CAPTURE_STALE_SECS=30 added next to CAPTURE_INTERVAL with a
note that it should be at least 2x the interval.

install.sh: generates sky-cam-watchdog-<cam>.service for every camera that
has a RTSP URL configured. The watchdog service starts after and alongside
the capture service, restarts on failure, and is enabled/started with the
same loop as the capture service.

README.md: note about watchdog and journalctl command for its logs.

https://claude.ai/code/session_01C4jbd3waXG3eKZYbGUjLUQ

install.sh

@@ -137,6 +137,26 @@ WantedBy=default.target
 EOF
         echo "  wrote sky-cam-capture-${cam}.service"
         capture_services+=("sky-cam-capture-${cam}")
+
+        cat > "$UNIT_DIR/sky-cam-watchdog-${cam}.service" <<EOF
+[Unit]
+Description=Sky-Cam — ${cam}: capture watchdog (stall detection)
+After=sky-cam-capture-${cam}.service
+Wants=sky-cam-capture-${cam}.service
+
+[Service]
+Type=simple
+ExecStart=$SCRIPT_DIR/capture-watchdog.sh ${cam}
+Restart=on-failure
+RestartSec=10
+StandardOutput=journal
+StandardError=journal
+
+[Install]
+WantedBy=default.target
+EOF
+        echo "  wrote sky-cam-watchdog-${cam}.service"
+        capture_services+=("sky-cam-watchdog-${cam}")
     else
         echo "  WARNING: CAM_RTSP_${cam} not set — skipping capture service for ${cam}"
         echo "           (set CAM_RTSP_${cam}=rtsp://... in sky-cam.conf to enable)"