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>

scripts/discover-devices.sh

@@ -0,0 +1,45 @@
+#!/bin/bash
+# Runs on the CT122 host (not in a container) via the homelab-monitor-discover
+# systemd timer. Real ARP entries live in the host's network namespace, not an
+# isolated Docker bridge network, so this has to run here rather than inside
+# the api container — see docs/device-discovery.md.
+#
+# Pings every address in the subnet (ICMP only, no port scanning) to populate
+# the ARP cache, then dumps ip+mac+state pairs as JSON. The API service reads
+# this file and does known/unknown labeling using config/hosts.yaml.
+set -euo pipefail
+
+SUBNET_PREFIX="${SUBNET_PREFIX:-192.168.1}"
+OUTPUT_FILE="${OUTPUT_FILE:-/opt/homelab-monitor/data/devices-raw.json}"
+IFACE="${IFACE:-eth0}"
+
+mkdir -p "$(dirname "$OUTPUT_FILE")"
+
+for i in $(seq 1 254); do
+  ping -c 1 -W 1 "${SUBNET_PREFIX}.${i}" >/dev/null 2>&1 &
+  # cap concurrency so we don't fork 254 pings at once
+  if (( i % 32 == 0 )); then wait; fi
+done
+wait
+
+TMP_FILE="$(mktemp)"
+{
+  echo "["
+  first=1
+  # `ip neigh show dev <iface>` output: "<ip> lladdr <mac> <state> [router]"
+  while read -r ip _ mac state _; do
+    [[ "$ip" == *:* ]] && continue  # skip IPv6 (link-local neighbor entries)
+    [[ -z "$mac" ]] && continue
+    case "$state" in
+      REACHABLE|STALE|DELAY|PERMANENT) ;;
+      *) continue ;;
+    esac
+    [[ $first -eq 0 ]] && echo ","
+    first=0
+    printf '{"ip":"%s","mac":"%s","state":"%s"}' "$ip" "$mac" "$state"
+  done < <(ip neigh show dev "$IFACE")
+  echo
+  echo "]"
+} > "$TMP_FILE"
+
+mv "$TMP_FILE" "$OUTPUT_FILE"