Instead of hardcoding '/irc' in two places, the Traefik path prefix is now
derived from the mediaProxy publicUrl_pathPrefix variable by stripping the
trailing slash (Traefik paths must not end with a slash, except for '/').

Added validation to ensure:
- publicUrl_pathPrefix starts and ends with a slash (required by the service)
- Traefik path prefix doesn't end with a slash (consistent with other roles)