Turn Reattach Stream

Re-attach to a turn that is already running (TOS-2090).

POST /messages/stream hands the reply to exactly one caller: the SSE stream IS that request's response body. Anyone who does not hold it - a pane React remounted when focus mode moved it, a reloaded page, a second tab, a second device - had no way to watch a turn in progress and saw the reply only once it finished and the assistant message landed.

This endpoint closes that gap by reading chat_stream_progress, which the running turn updates on a coalesced flush. It emits:

• snapshot - the accumulated text so far, so a late joiner catches up in one frame rather than starting from blank;
• progress - whenever any agent's seq advances or the visible agent row set changes. The payload carries the FULL current set rather than a delta, so a dropped or duplicated frame cannot desynchronise the bubbles;
• idle - the turn is over (or there was never one). The client closes and reconciles from /messages, where the outcome now lives.

Polling Postgres rather than subscribing in-process is deliberate: awen-api runs several Cloud Run pods and the re-attaching client rarely lands on the pod running the turn.