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>

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