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.

backend/internal/latest/sites.go

@@ -9,6 +9,7 @@ import (
 	"sort"
 	"strconv"
 	"strings"
+	"time"
 
 	"github.com/chromedp/chromedp"
 )
@@ -32,6 +33,11 @@ type site struct {
 	LatestChapter func(seriesURL, body string) (latestChapter, bool)
 	// Cover finds the Cover address in a fetched body.
 	Cover func(seriesURL, body string) (string, bool)
+	// Rest is how long a Series of this Site rests between Polls.
+	Rest time.Duration
+	// Gap is the Lane's strictest pace: at least one second must pass between
+	// two consecutive Series-page Polls of this Site (issue #100).
+	Gap time.Duration
 	// Browser reads this Site's payload from a cleared browser tab; nil
 	// means the page is fetched over plain TLS.
 	Browser *browserRead
@@ -380,6 +386,50 @@ func publishedCoverURL(value string) string {
 	return strings.ReplaceAll(value, " ", "%20")
 }
 
+// Poll Lane constants (issue #100). The per-Site structure is deliberately
+// uniform at first — every Site rests an hour and gaps ten seconds — but it
+// exists so a single Site can be slowed if it turns hostile, and the numbers
+// stay in the registry so the structure has a place to differ.
+const (
+	// defaultRest is how long every Series rests between Polls.
+	defaultRest = time.Hour
+	// defaultGap is the strictest pace of every Lane unless the eligible
+	// Series count forces it tighter.
+	defaultGap = 10 * time.Second
+	// minGap floors the effective gap. One request per second is already an
+	// order of magnitude past the strictest rate rule a free-plan Site can
+	// express (docs/research/cloudflare-bot-scoring-and-poll-cadence.md);
+	// below it the Lane is outrunning its own plan and says so loudly.
+	minGap = time.Second
+	// refuseBackoff is how long a Lane waits after its Site refused twice in
+	// one run before attempting it again.
+	refuseBackoff = 15 * time.Minute
+	// browserWakeCount and browserWakeAge gate a browser Lane's run: five or
+	// more due Series, or any one of them waiting this long, or Chrome stays
+	// asleep (ADR-0005 on-demand browser).
+	browserWakeCount = 5
+	browserWakeAge   = 15 * time.Minute
+)
+
+// effectiveGap is a Site's pace: the registry gap, or one rest divided by the
+// eligible Series count when that is smaller, never below one second. The
+// denominator follows defaultRest rather than a literal hour so a Site whose
+// rest is ever changed keeps its per-Series pace in step. The second return is
+// true when the one-second floor engaged (and the Lane logs a warning naming
+// the Site, every round it does).
+func effectiveGap(s site, eligible int) (time.Duration, bool) {
+	gap := s.Gap
+	if eligible > 0 {
+		if perSeries := defaultRest / time.Duration(eligible); perSeries < gap {
+			gap = perSeries
+		}
+	}
+	if gap < minGap {
+		return minGap, true
+	}
+	return gap, false
+}
+
 // sites is the registry: one entry per Site, keyed by the stored site string.
 // Adding a Site means adding an entry here and nowhere else — the dispatch
 // functions above and the poller's route list are lookups into this map. An
@@ -390,16 +440,22 @@ var sites = map[string]site{
 		Host:          "asurascans.com",
 		LatestChapter: asuraLatestChapter,
 		Cover:         ogImageCover,
+		Rest:          defaultRest,
+		Gap:           defaultGap,
 	},
 	"demonic": {
 		Host:          "demonicscans.org",
 		LatestChapter: demonicLatestChapter,
 		Cover:         ogImageCover,
+		Rest:          defaultRest,
+		Gap:           defaultGap,
 	},
 	"comix": {
 		Host:          "comix.to",
 		LatestChapter: comixLatestChapter,
 		Cover:         comixCoverEntry,
+		Rest:          defaultRest,
+		Gap:           defaultGap,
 		Browser: &browserRead{
 			Read: comixRead,
 			// The interstitial is served in place of the page, so "arrived"
@@ -414,6 +470,8 @@ var sites = map[string]site{
 		Host:          "kagane.to",
 		LatestChapter: kaganeLatestChapter,
 		Cover:         kaganeCoverEntry,
+		Rest:          defaultRest,
+		Gap:           defaultGap,
 		Browser: &browserRead{
 			Read: kaganeRead,
 			Done: func(body string) bool { return body != "" },
@@ -426,6 +484,8 @@ var sites = map[string]site{
 		Host:          "novelfull.com",
 		LatestChapter: novelfullLatestChapter,
 		Cover:         novelfullCoverEntry,
+		Rest:          defaultRest,
+		Gap:           defaultGap,
 		Browser: &browserRead{
 			Read: novelfullRead,
 			// The interstitial has a DOM too, so "the payload arrived" has to
@@ -438,13 +498,15 @@ var sites = map[string]site{
 		Host:          "lightnovelworld.net",
 		LatestChapter: lnwLatestChapter,
 		Cover:         ogImageCover,
+		Rest:          defaultRest,
+		Gap:           defaultGap,
 	},
 }
 
 // browserBackedSites is derived from the registry: the Sites whose pages are
-// read through the browser sidecar, which are also the ones granted the longer
-// cooldown. Sorted so callers that range it (the due query, the browser
-// fetcher's dispatch) see a stable order instead of map-iteration noise.
+// read through the browser sidecar. Sorted so callers that range it (the
+// browser fetcher's dispatch) see a stable order instead of map-iteration
+// noise.
 func browserBackedSites() []string {
 	out := make([]string, 0, len(sites))
 	for name, s := range sites {