Set Translation Mode

Operator-facing translator mode for ONE call — the authoritative Off.

paused is a hold: it suppresses the turn but the translator stays armed, and an operator who wants translation to STOP has no way to say so from the call, because what armed it is a SAVED config (L1 thread.voice_translation, else L0 channel_account default) that the header never mentions. mode='off' writes the same mode_override the agent's stop_translation tool writes, and TranslatorState.from_runtime lets a valid override beat cfg.mode — including "off" on a chat configured 'voice'. No MT, no subtitle, no TTS.

mode=null clears the override, so the call goes back to whatever the saved config says rather than to a hardcoded on. That is the honest undo: the operator turned Off for this call, not for the chat (that edit lives in the thread's Translation settings).

Same runtime hash and the same workspace gate as /pause — this is not a new mechanism, only the missing entry point. The worker-called handover endpoint stays at /internal/voice/translation/{id}/mode-override (prod workers reach the gateway through an /internal/-only shim), and it carries voice-continuity fields this one deliberately does not touch.