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.

CONTEXT.md

@@ -86,7 +86,7 @@ _Avoid_: client report, user poll, observation, claim
 
 **Acquisition**:
 The single read of a Series page made the moment the Series first exists, giving it
-both its Latest Chapter and its Cover without waiting out the Poll queue. Distinct
+both its Latest Chapter and its Cover without waiting for the Lane's pace. Distinct
 from a Poll in the two ways that matter: a Reader is present — it is triggered by
 their first Bookmark of that Series — and it is the only read that establishes a
 Cover rather than refreshing facts. It happens once in a Series's life; every later