When an assistant message sits status='streaming' with no token activity
for 60+ seconds, the chat shows a banner above the input offering Retry
or Discard. Both clear the stale row via a new backend endpoint
POST /api/chats/:id/discard_stale that updates status='failed' and
publishes chat_status='idle'.
Closes the UX gap that caused the 2026-05-21 debugging spiral —
slow streams and dead streams now look different to the user.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
