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>

.env.example

@@ -73,26 +73,13 @@ DISCORD_REDIRECT_URI=
 # The backend re-checks each bookmarked series' newest published chapter on its
 # own schedule, so latest_chapter stays fresh even when you never open the manga
 # sites. This runs in parallel with the userscript's own in-browser check.
-# Set to 0 to turn it off entirely.
+# Set to 0 to turn it off entirely. Pace is per Site (one Poll Lane per Site,
+# issue #100) and lives in the backend registry, not here — there is nothing
+# else to configure.
 # LATEST_CHAPTER_POLL_ENABLED=1
 #
-# Two independent clocks. COOLDOWN is how long a plain-TLS series rests between
-# checks; BROWSER_COOLDOWN is the longer rest for kagane and novelfull. INTERVAL
-# is how often the poller wakes up and looks for series past their cooldowns.
-# Shortening INTERVAL cannot shorten either cooldown.
-LATEST_CHAPTER_POLL_COOLDOWN=1h         # plain-TLS per series, floor 15m
-LATEST_CHAPTER_POLL_BROWSER_COOLDOWN=6h # browser-backed per series, floor 15m
-LATEST_CHAPTER_POLL_INTERVAL=10m        # how often to wake
-LATEST_CHAPTER_POLL_BATCH=14            # series per wake
-LATEST_CHAPTER_POLL_STAGGER=20s         # delay between fetches in a batch
-#
-# Uses a ticker, not an immediate first run: the first poll happens one
-# INTERVAL after startup, not at startup. A container restarting more often
-# than INTERVAL never polls.
-#
-# BATCH x (COOLDOWN / INTERVAL) series hold the cooldown cadence — 84 with these
-# defaults. Beyond that the cadence stretches uniformly rather than breaking;
-# raise BATCH or lower INTERVAL. Keep BATCH x STAGGER under INTERVAL.
+# Every Site rests an hour between checks and gaps ten seconds between fetches;
+# a Site with many Series tightens its own gap. See backend/internal/latest/sites.go.
 
 # CDP endpoint of the browser, used for the two sites behind a Cloudflare
 # JavaScript challenge (kagane, novelfull) and by the web UI's kagane cover