feat: add MQTT sensor publishing for Home Assistant

Adds a new s6 service (svc-foldingathome-mqtt) that polls the FAH 8 API
on port 7396 every 30 seconds and publishes 4 sensors to HA via MQTT
autodiscovery: Status, PPD, Work Unit Progress, and Credit estimate.

Requires the Mosquitto broker add-on; declared as mqtt:want so the
add-on remains functional without it.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Dockerfile

@@ -30,6 +30,7 @@ RUN \
         curl \
         jq \
         libexpat1 \
+        mosquitto-clients \
         netcat-openbsd && \
     echo "**** install foldingathome ****" && \
     if [ "${BUILD_ARCH}" = "aarch64" ]; then \