Long-run chat: persist user turns, recover from proxy disconnects

Three defects surfaced by a 13-minute, 31-document project-chat review
that ended in a bare "Sorry, something went wrong" while the answer sat
finished in the database.

1. No user message was ever saved. Both chat routes inserted a
   `workflow` field into chat_messages, which has no such column;
   PostgREST rejected every insert and the unchecked result hid it. Drop
   the field and check the error. Reloaded chats now show the questions,
   not just the answers.

2. Cloudflare caps a proxied request at ~10 minutes no matter how often
   the SSE heartbeat fires, so a long agentic run loses its client while
   the backend keeps going and persists the turn. The client now says so
   and polls the saved chat until the answer lands, instead of leaving a
   dead error in place.

3. The client ignored the SSE `error` event entirely, so a server-side
   failure ended the stream as a blank assistant turn. Surface it (and
   send the real message rather than "Stream error").

Also add start/disconnect/finish logging to the project chat route,
which had none - the reason this run left almost no trace.