Send `X-HeyGen-Client-Source: media-use` on every media-use HeyGen API
request (both auth types, via heygenAuthHeaders + the heygenJSON transport),
so backend billing meta can isolate media-use consumption from other free
TTS and avatar-video usage. Unconditional of auth type — a paying user's
media-use call is still media-use — unlike the OAuth-only cli-source header
that gates the free allowance.