feat: availability broadcast (Kind 30078) + WiFi auto-connect

Wire up the availability broadcast composable to publish the ATM's
status as a replaceable Kind 30078 Nostr event. Publishes on
availability change (debounced) and as a 5-minute heartbeat so
monitors can detect offline machines.

Also adds WiFi auto-connect for BATM3: reads SSID/PSK from
/var/lib/lamassu-atm/wifi.conf at boot. Credentials stay local.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

deploy/nixos/hardware/batm3.nix

@@ -68,6 +68,43 @@
     hybrid-sleep.enable = false;
   };
 
+  # WiFi: read credentials from /var/lib/lamassu-atm/wifi.conf (not in repo)
+  # Format: SSID=MyNetwork\nPSK=MyPassword
+  system.activationScripts.wifi-setup = ''
+        WIFI_CONF="/var/lib/lamassu-atm/wifi.conf"
+        if [ -f "$WIFI_CONF" ]; then
+          SSID=$(grep '^SSID=' "$WIFI_CONF" | cut -d= -f2-)
+          PSK=$(grep '^PSK=' "$WIFI_CONF" | cut -d= -f2-)
+          if [ -n "$SSID" ] && [ -n "$PSK" ]; then
+            CONN_FILE="/etc/NetworkManager/system-connections/$SSID.nmconnection"
+            if [ ! -f "$CONN_FILE" ]; then
+              cat > "$CONN_FILE" << EOF
+    [connection]
+    id=$SSID
+    type=wifi
+    autoconnect=true
+
+    [wifi]
+    mode=infrastructure
+    ssid=$SSID
+
+    [wifi-security]
+    key-mgmt=wpa-psk
+    psk=$PSK
+
+    [ipv4]
+    method=auto
+
+    [ipv6]
+    method=auto
+    EOF
+              chmod 600 "$CONN_FILE"
+              echo "[WiFi] Created connection for $SSID"
+            fi
+          fi
+        fi
+  '';
+
   # eGalax touchscreen (Dell 9030 AIO built-in panel)
   # The eGalax HID descriptor confuses libinput (treats it as touchpad).
   # Fix: unbind from usbhid at boot, bind to usbtouchscreen kernel module,