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>

config.yaml

@@ -13,6 +13,8 @@ ports_description:
   7396/tcp: Folding@home Web Interface
 map:
   - config:rw
+services:
+  - mqtt:want
 options:
   user: ""
   team: "247478"