Send Agent Chat Message Async

Durable (fire-and-forget) chat turn: enqueues an async_chat_turn background job instead of holding a connection open. For callers that can't reliably hold an SSE stream open long enough to see it through (e.g. a mobile notification-tray reply from a backgrounded app) — the worker drives the turn via the same create_agent_chat_stream path the SSE route uses, so the reply lands (and pushes) even if the caller goes away immediately after the 202.

Idempotent via the shared X-Idempotency-Key header infra (see security/http_idempotency.py, same mechanism as /api/create-thread and /api/analyze-receipt): iOS can redeliver a notification action, and a client-timed-out POST may have already landed server-side. Requests without the header keep today's at-least-once behavior unchanged.