Fix background task auto-resume invisible in UI

[pufit]

Summary

• Fix auto-resume response after background tasks being invisible to the user
• Add auto_resume_start WebSocket event broadcast before auto-resume to reliably enter streaming mode
• Remove !isStreaming guard from session_running handler (now idempotent)
• Track background watcher tasks for proper cancellation on stop/shutdown
• Touch session during polling to prevent idle sweep killing the client
• Await auto-resume instead of fire-and-forget create_task for error propagation

Test plan

• Trigger a background task — verify thinking cursor appears when task completes and response streams visibly
• Type during background task polling — verify no token loss or duplicate responses
• Stop session while background task runs — verify watcher is cancelled cleanly
• Wait >5min with background task — verify idle sweep doesn't kill client
• All 326 existing tests pass
• Frontend builds clean

Generated by Nerve