End Conversation

End a conversation at the visitor's request (Quick Exit).

Marks the conversation completed and triggers the post-processing pipeline. Idempotent — ending an already-completed conversation returns ended=false.

Auth: X-Session-Token header, or a JSON body {"session_token": ...} as a fallback for navigator.sendBeacon, which cannot set headers and must send text/plain to avoid a CORS preflight — so the body is parsed manually rather than via a Pydantic model.