Unscope lnw chapter scan, truncate at comment thread (#87)

The Poll scoped the lightnovelworld scan to the slug derived from the
stored address, so a Series publishing chapters under more than one
Chapter Slug froze its Latest Chapter. The branch now matches any
chapter-shaped address on the host (lnwChapterRe) and scans only the
body before the wpdiscuz thread marker (wpd-threads); a body without
the marker is skipped and logged with its length, never scanned whole.
The size-cap comment now states the measured ~3.5x headroom.

backend/internal/latest/sites.go

@@ -3,6 +3,7 @@ package latest
 import (
 	"encoding/json"
 	"html"
+	"log"
 	"net/url"
 	"regexp"
 	"strconv"
@@ -58,11 +59,26 @@ var kaganeChapterRe = regexp.MustCompile(`"chapter_no":"([0-9.]+)"`)
 // "/<slug>/chapter-<n>[-<title-slug>].html". Verified live 2026-08-05.
 var novelfullSlugRe = regexp.MustCompile(`^/([^/?#]+)\.html$`)
 
-// lnwSlugRe does the same for lightnovelworld, whose series pages live under
-// /novel/<slug>/ while its chapter URLs are flat at the site root:
-// "/<slug>-chapter-<n>/", absolute in the page's own anchors. Verified live
-// 2026-08-05.
-var lnwSlugRe = regexp.MustCompile(`^/novel/([^/?#]+)/?$`)
+// lnwChapterRe matches any chapter-shaped address on lightnovelworld. Unlike
+// asura, novelfull and comix — which scope to their stored series slug so a
+// foreign chapter link cannot contribute — this Site's chapter addresses carry
+// the Chapter Slug, which is not the Series identity: one Series may publish
+// under several Chapter Slugs (measured 2026-08-11: a sampled novel serves
+// 1-99 under one slug and 100-423 under another), so no stored-slug pattern can
+// cover a Series' whole list. An unscoped match is safe because
+// latestChapterFrom truncates the body at the comment thread before scanning
+// (lnwCommentMarker); without that, a visitor's comment could set the Latest
+// Chapter on the shared Series row.
+var lnwChapterRe = regexp.MustCompile(`lightnovelworld\.net/[a-z0-9-]+-chapter-([0-9.]+)/`)
+
+// lnwCommentMarker is the boundary of lightnovelworld's server-rendered
+// wpdiscuz comment thread. It occurs exactly once per page and follows every
+// chapter anchor (measured 2026-08-11,
+// docs/research/lightnovelworld-chapter-vs-series-slug.md §6), so cutting the
+// body at its first occurrence keeps the whole chapter list while excluding a
+// region any visitor can write to. Absent means the page shape changed: the
+// body is skipped, never scanned whole.
+const lnwCommentMarker = "wpd-threads"
 
 // latestChapterFrom returns the highest chapter number body advertises for this
 // series. ok is false when the body yields nothing usable — an unknown site, an
@@ -82,6 +98,11 @@ var lnwSlugRe = regexp.MustCompile(`^/novel/([^/?#]+)/?$`)
 // such guarantee — demonicChapterRe matches any chaptered.php?manga=<id> anchor
 // with no per-series scoping, because the stored series_id for demonic is a
 // slug, not the numeric id the URL carries, so it cannot easily be scoped.
+// lightnovelworld is unscoped for a stronger reason: its chapter addresses
+// carry the Chapter Slug, which is not the Series identity, so no stored-slug
+// pattern can cover a Series' whole list (see lnwChapterRe). Instead of
+// scoping, its branch scans only the body before the comment thread marker and
+// skips the page when the marker is absent.
 func latestChapterFrom(site, seriesURL, body string) (latestChapter, bool) {
 	var re *regexp.Regexp
 	switch site {
@@ -126,15 +147,19 @@ func latestChapterFrom(site, seriesURL, body string) (latestChapter, bool) {
 		// and neither may contribute to the maximum.
 		re = regexp.MustCompile(`/` + regexp.QuoteMeta(m[1]) + `/chapter-([0-9.]+)`)
 	case "lightnovelworld":
-		u, err := url.Parse(seriesURL)
-		if err != nil {
+		// The comment thread below the chapter list is the one region of the
+		// page any visitor can write to, so the scan never reads past it (see
+		// lnwChapterRe). A body without the marker is skipped, never scanned
+		// whole — a redesign must degrade into staleness, not into a wrong
+		// shared value. The body length distinguishes a genuine markup change
+		// from a body the size cap cut short.
+		i := strings.Index(body, lnwCommentMarker)
+		if i < 0 {
+			log.Printf("latest poll %q: no %s marker in %d bytes; skipping", seriesURL, lnwCommentMarker, len(body))
 			return latestChapter{}, false
 		}
-		m := lnwSlugRe.FindStringSubmatch(u.Path)
-		if m == nil {
-			return latestChapter{}, false
-		}
-		re = regexp.MustCompile(`lightnovelworld\.net/` + regexp.QuoteMeta(m[1]) + `-chapter-([0-9.]+)/`)
+		body = body[:i]
+		re = lnwChapterRe
 	default:
 		return latestChapter{}, false
 	}