feat: one Poll Lane per Site, replacing the shared pace (#100) (#106)

Closes #100.

Each Site runs its own Poll Lane: an independent goroutine with its own rest
and pace from the registry (`backend/internal/latest/sites.go`), replacing the
shared cooldown/interval/stagger/batch configuration. Rest (1h, all six Sites
including the browser trio) is enforced by the due query's WHERE clause; the
Lane sleeps its effective gap between fetches — the registry 10s, or
rest/eligible when a Site holds enough Series, floored at 1s with a
Site-naming warning when the floor engages.

Lane-local failure handling:
- Two challenge-held results stop that Site's Lane for 15m; the probes keep
  their stamp, untried Series stay due.
- A lost browser sets a shared Poller flag: the other browser Lanes skip
  their passes for the same 15m (no stamp-per-pass-per-Lane on a dead tab),
  then decay and probe again.
- Browser wake gate preserved (5 due, or one waiting 15m, ADR-0005); one tab
  shared by the three browser Sites; "browser lane behind by X" logged every
  pass.
- Cover work (healing a stored source URL and filling a blank from the series
  page) runs in the background so a slow CDN cannot consume a Lane's gap.

Removed: `LATEST_CHAPTER_POLL_{COOLDOWN,BROWSER_COOLDOWN,INTERVAL,BATCH,STAGGER}`
and the 6h browser rest. Only `LATEST_CHAPTER_POLL_ENABLED` remains; DEPLOY.md
documents the exact `.env` edit. ADR-0010 records the decisions.

Reviewed-on: #106
Co-authored-by: Sulthan Zaki <sultankiki05@gmail.com>
Co-committed-by: Sulthan Zaki <sultankiki05@gmail.com>

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