chore(start): mount /etc/caddy/sites into container for upstream watcher (DC-049 fixup)

The DC-049 dead-upstream watcher reads reverse_proxy host directives from
/etc/caddy/sites/*. Bind-mount the directory into the container so the
in-container watcher can see what the host's Caddy is configured to proxy.
Without this mount the watcher would see zero sites and silently no-op.

start.sh

@@ -146,6 +146,7 @@ docker run -d --restart unless-stopped --name ${CONTAINER_NAME} \
   -v ${DATA_DIR}:/app/data \
   -v ${BACKUPS_DIR}:/app/backups \
   -v ${CADDYFILE}:/caddyfile \
+  -v /etc/caddy/sites:/etc/caddy/sites:ro \
   -v /var/run/docker.sock:/var/run/docker.sock \
   -v ${ASSETS_DIR}:/app/assets \
   -v ${UPDATES_DIR}:/app/updates \