Finalizes an UploadJob whose R2 object is already fully present and
verified, transitioning it to the distributing/completed state and
kicking off distribution to the draft's target platforms.
Parameters
jobId: string
UploadJob id, already confirmed to exist and be owned
by the caller.
userId: string
Owning user id, for downstream authorization checks.
Returns Promise<{distributing:boolean}>
Whether distribution was actually started (mirrors the
existing distributing field in the complete route's response).
Finalizes an UploadJob whose R2 object is already fully present and verified, transitioning it to the distributing/completed state and kicking off distribution to the draft's target platforms.