old json.parse(default=...) silently failed on every track change so
recent-played always showed only one entry. modern let-form parses
the existing history file correctly.
- fix config paths from icecast2 to icecast (correct for this image)
- fix mount target to /etc/icecast.xml (not /etc/icecast2/icecast.xml)
- replace wget healthcheck with bash /dev/tcp (wget absent in container)