v0.5.0 makes LIVEKIT_FULL_ACCESS_HOMESERVERS a required setting and
drops the implicit `*` wildcard default upstream.

Split the full-access-homeservers list into _default/_auto/_custom
parts (following the convention used for other variables in this role),
with a sane _default of the homeserver's own domain. This also lets
group_vars/matrix_servers drop its now-redundant override.

Add a validate_config.yml check requiring the setting to be defined.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>