feat: one Poll Lane per Site, replacing the shared pace (#100)

Each Site runs its own poll goroutine, paced by Rest and Gap from the
registry (sites.go) instead of the shared cooldown/interval/stagger/batch
config. Rest is enforced by the due query's WHERE clause; the Lane sleeps
its effective gap between fetches (hour/eligible, floored at 1s).

Lane-local failures: two refusals stop that Site for 15m, a lost browser
stops only the round's remaining browser Lanes, and cover heals moved to
background goroutines so a slow CDN cannot consume a Lane's gap. The five
LATEST_CHAPTER_POLL_* pace env vars are gone; only the kill switch
remains.

backend/internal/latest/browser.go

@@ -18,7 +18,7 @@ import (
 
 // challengeTimeout bounds one navigate-and-solve. A Cloudflare managed
 // challenge clears in a few seconds when it clears at all; anything longer is a
-// challenge that is not going to pass, and the caller's cooldown was already
+// challenge that is not going to pass, and the caller's rest was already
 // stamped before this ran.
 const challengeTimeout = 45 * time.Second
 
@@ -213,7 +213,7 @@ func (f *BrowserFetcher) Image(ctx context.Context, imageURL string) ([]byte, st
 
 // errChallengeHeld reports that the budget ran out with the interstitial still
 // up. Distinct from a transport failure: it means "this site said no", which
-// the poller answers with a 403 and its ordinary cooldown.
+// the poller answers with a refusal backoff for that Site's Lane (issue #100).
 var errChallengeHeld = errors.New("challenge held")
 
 // errBrowserInterrupted distinguishes a remote Chrome restart from the