Add LAN device discovery (ping sweep + ARP, known/unknown labeling)

Runs as a host-level systemd timer on CT122 (scripts/discover-devices.sh)
rather than inside the api container, since real ARP entries live in the
host's network namespace, not Docker's bridge network. See
docs/device-discovery.md for the full writeup, including why literal
passive-only ARP reading was dropped (near-empty result in practice).

API reads the resulting JSON file each poll cycle, cross-references
config/hosts.yaml's knownDevices list by IP, and serves /api/devices.
Dashboard gets a new "Network Devices" table.

Closes #9.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

deploy/systemd/homelab-monitor-discover.service

@@ -0,0 +1,10 @@
+[Unit]
+Description=homelab-monitor LAN device discovery (ping sweep + ARP read)
+After=network-online.target
+
+[Service]
+Type=oneshot
+ExecStart=/opt/homelab-monitor/scripts/discover-devices.sh
+Environment=SUBNET_PREFIX=192.168.1
+Environment=OUTPUT_FILE=/opt/homelab-monitor/data/devices-raw.json
+Environment=IFACE=eth0