Set/clear the org-level channel-prompt override map

PUT body for the org-level channel-prompt override map.

Replaces the whole map (last-writer-wins), matching the judge-model settings semantics. Sparse: a channel absent from channel_prompts inherits the platform default. Keyed by :data:OverridableChannel — those are the only channels with a platform medium block to override; any other channel (api, sandbox, or an unknown value) rejects with 422.

The org's currently stored channel-prompt override map (empty when unset).