Submit post-conversation feedback from the platform (authenticated).

Staff-facing counterpart to the public widget feedback endpoint: lets an authenticated user record the agent's configured feedback form for a conversation that has no widget of its own (e.g. a phone call). The caller supplies widget_id — the widget whose form defines the accepted fields — which must belong to the same organization (enforced implicitly by the org-scoped DB) and the same agent as the conversation. Stored under custom_metadata.feedback_form; unlike the public widget path, resubmitting overwrites the existing feedback so reviewers can correct or extend it while auditing a transcript.