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

Dockerfile

@@ -1,6 +1,9 @@
-FROM python:alpine3.17
+FROM python:3.12-alpine
+RUN apk add --no-cache iw
 WORKDIR /app
+COPY requirements.txt /app/
+RUN pip install --no-cache-dir -r requirements.txt
 COPY . /app
-RUN pip install -r requirements.txt
 EXPOSE 9999
+VOLUME /data
 ENTRYPOINT ["python", "app.py"]