## Problem
Race countdown had three reliability issues:
- Notified every second regardless of remaining time (noisy for
hours-long waits)
- Never re-fetched start times (stale if contest rescheduled)
- No retry on setup failure at T=0 — if the scraper failed at the most
critical moment, the entire countdown was wasted
## Solution
- **Notification tiers**: >1h every 15m, >5m every 1m, >1m every 10s,
≤60s every 1s
- **Drift correction**: re-fetch contest list every 10 minutes, update
`start_time` if changed
- **T=0 retry**: `race_try_setup` calls `scrape_contest_metadata` with
the new `on_error` callback, retrying up to 15 times at 3s intervals
(~45s). Caches metadata on success, then calls `setup_contest` on the
fast (cached) path. Token guard invalidates stale retries after
cancellation.
- **`on_error` for `scrape_contest_metadata`**: optional 4th param,
backward-compatible
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