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.

.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