Add network monitoring, dashboard, and Home Assistant MQTT publishing

Keep /alive unchanged and serve a dashboard at / with status cards,
latency graphs, an event timeline, and a Wi-Fi section.

Background monitors on configurable intervals: TCP reachability,
DNS resolution, public IP change detection, host reboot detection,
Wi-Fi link quality, and Wi-Fi scan of networks in range via iw.
The Wi-Fi interface is selected with WIFI_INTERFACE; the page lists
visible interfaces and reports when none is available. Scan results
stay in memory, other metrics go to SQLite on a mounted volume.

Optional MQTT publishing with Home Assistant discovery groups all
sensors under one device per host.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TdvJYoY8kc8bmBck89U2i6

docker-compose.yml

@@ -0,0 +1,23 @@
+services:
+  healthcheck:
+    build: .
+    image: healthcheck-container
+    container_name: healthcheck
+    restart: unless-stopped
+    # Host networking exposes the Wi-Fi interface to the container. The app
+    # listens on port 9999 on the host directly, so no `ports:` mapping is used.
+    network_mode: host
+    cap_add:
+      - NET_ADMIN
+    volumes:
+      - healthcheck-data:/data
+    environment:
+      DEVICE_NAME: pi-livingroom
+      WIFI_INTERFACE: wlan0
+      WIFI_SCAN_INTERVAL: "60"
+      # MQTT_HOST: homeassistant.local
+      # MQTT_USERNAME: mqtt
+      # MQTT_PASSWORD: secret
+
+volumes:
+  healthcheck-data: