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>

DEPLOY.md

@@ -93,6 +93,14 @@ the backend dials but not the password the database expects, and `bookmark-api`
 crash-loops on `password authentication failed`. Set it before §2 and leave it
 alone.
 
+An `.env` written before issue #100 carries the old poll-pace names
+(`LATEST_CHAPTER_POLL_COOLDOWN`, `_BROWSER_COOLDOWN`, `_INTERVAL`, `_BATCH`,
+`_STAGGER`). All five are dead configuration now — the pace lives in the Site
+registry (`backend/internal/latest/sites.go`), so **delete those lines** and
+keep only the kill switch `LATEST_CHAPTER_POLL_ENABLED`. Leaving them behind
+is harmless (nothing reads them) but silently misleads the next person who
+edits the file.
+
 > Match `TRAEFIK_ENTRYPOINT` / `TRAEFIK_CERTRESOLVER` to your Traefik's actual
 > names (check your Traefik static config — common alternatives: `https`,
 > `myresolver`, `cloudflare`). Wrong names = no certificate issued.
@@ -505,7 +513,7 @@ picks up a restarted Chrome's new debugger UUID by itself.
 | kagane rows never get a `latest_chapter`; log says `browser fetcher disabled` or nothing at all | `BROWSER_WS_URL` unset. Expected before §7 is done. |
 | kagane polls all fail; log shows a 500 from `/json/version` | `BROWSER_WS_URL` names a MagicDNS hostname (or any name). Chrome's DevTools handler only accepts an IP or `localhost` — use the tailnet IP. |
 | kagane polls fail with a connection error | Home machine off, off the tailnet, or the unit is down. `tailscale ping <machine>`, then `docker compose ps` in its `chrome/`. Costs freshness only; stored covers keep serving. |
-| kagane cover is a placeholder for a newly bookmarked series | Its cover has never been fetched and the browser is unreachable. It fills in on the next successful poll of that series (up to `LATEST_CHAPTER_POLL_BROWSER_COOLDOWN`, default 6h). |
+| kagane cover is a placeholder for a newly bookmarked series | Its cover has never been fetched and the browser is unreachable. It fills in on the next successful poll of that series. |
 | `compose` in `chrome/` errors `set BROWSER_BIND_ADDR to this machine's tailnet IP` | No `chrome/.env`, or the variable is empty. Deliberate — it has no default so an unset value cannot publish CDP to the LAN. |
 | browser container restarts, or is OOM-killed | `docker inspect bookmark-browser --format '{{.RestartCount}} {{.State.OOMKilled}}'`. The 512 MiB cap is sized against a measured 645 MiB untuned peak; a real breach is a Chrome regression worth reading `docker logs` for, not a number to raise reflexively. |