Each auth.Provider now declares DefaultServiceURL() so callers do not
need to know service-specific endpoints. ApplyAuthDefaults fills Engine
and URL from the provider before validation runs — explicit flags always
win, and providers with no default URL require -url to be set explicitly.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Lay the groundwork for splitting service-specific logic (WB / Jazz /
Telemost API flows) from wire-level transport engines (LiveKit, Goolom,
future Jitsi). An engine takes only URL+Token+Name+network knobs; an
auth provider produces those credentials and reports which engine it
feeds. RoomCreator is an optional capability for the gen mode.
Existing carriers and providers are untouched.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
