Make retention delete fast and show progress

[adn8naiagent]

A first sweep over a large backlog appeared to hang: delete_session made about five sequential R2 round trips per session, so deleting 134 sessions meant ~670 calls held open behind one request with nothing on screen. Measured against the bucket, 100 objects one at a time took 24.5s; the same 100 in bulk take 0.9s.

• storage.keys_under lists once from the shared root; delete_keys removes exactly those keys in batches of 1000, reporting progress per batch
• sweep uses them instead of per-session deletes, and skips tombstones: they exist to stop auto_precompute re-downloading a removed session, and it only looks at the last 7 days, which retention never touches
• the sweep now runs in the background via start_sweep, with GET /api/storage/retention/status for progress, so the request no longer blocks and a refresh mid-delete doesn't lose it
• the panel polls that status and shows a progress bar over files deleted
• the retention controls stay hidden until usage has loaded, so a threshold can't be set against a total that hasn't arrived