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.

docker-compose.yml

@@ -58,14 +58,11 @@ services:
       # Second script from the same bindmount; the novel library is a separate
       # Violentmonkey install.
       NOVEL_USERSCRIPT_PATH: ${NOVEL_USERSCRIPT_PATH:-/userscript/novel-bookmark.user.js}
-      # Latest-chapter poller. LATEST_CHAPTER_POLL_ENABLED=0 in .env is the kill
-      # switch; it only takes effect because these are listed here.
+      # Latest-chapter poller. LATEST_CHAPTER_POLL_ENABLED=0 in .env is the
+      # kill switch; it only takes effect because it is listed here. Pace is
+      # per Site in the registry (one Poll Lane per Site, issue #100) — the
+      # cooldown/interval/stagger/batch knobs are gone with the shared pace.
       LATEST_CHAPTER_POLL_ENABLED: ${LATEST_CHAPTER_POLL_ENABLED:-1}
-      LATEST_CHAPTER_POLL_COOLDOWN: ${LATEST_CHAPTER_POLL_COOLDOWN:-1h}
-      LATEST_CHAPTER_POLL_BROWSER_COOLDOWN: ${LATEST_CHAPTER_POLL_BROWSER_COOLDOWN:-6h}
-      LATEST_CHAPTER_POLL_INTERVAL: ${LATEST_CHAPTER_POLL_INTERVAL:-10m}
-      LATEST_CHAPTER_POLL_BATCH: ${LATEST_CHAPTER_POLL_BATCH:-14}
-      LATEST_CHAPTER_POLL_STAGGER: ${LATEST_CHAPTER_POLL_STAGGER:-20s}
       # CDP endpoint for sites behind a JavaScript challenge (kagane,
       # novelfull). The browser is not part of this stack — it runs on the home
       # machine as its own unit (chrome/docker-compose.yml) and is reached over