fix(traces): fix rate limit backoff in ExternalFetcher

[gutonosa]

Rate limit backoff in ExternalFetcher never actually waits.

tokio::time::interval() fires its first tick immediately, so after a 429 the loop just takes the interval out, sees Ready, drops it, and retries with no delay. Cycle repeats on every rate limit hit.

What happens:

1. 429 → sets backoff = Some(interval(5s))
2. Next loop: .take() pulls it out, poll_tick() → instant Ready, if-let falls through, interval dropped
3. queue_next_reqs() fires immediately → another 429 → repeat

Fix: interval_at(now + timeout, timeout) so the first tick respects the delay.