feat(userscript): render Covers from the public route (#60)

Both userscripts stop scraping Covers and stop putting one on the wire.
A scraped address has no rendering path left now that the backend
acquires, stores and serves every Cover from its own origin (ADR-0007),
and keeping one would reintroduce third-party URLs into exactly the
place #47 came from.

- Adapters no longer read og:image / meta[name=image], and comix's
  img[alt] cover scan and novelfull's metaName helper are deleted.
- apiPut strips `cover` off every outgoing body, so a cached row's
  address (already ours) never travels back either. The server ignores
  the field regardless.
- Both card renderers swap in the existing `.cover.ph` placeholder when
  the image fails to load, so a failure looks designed rather than
  broken - the other half of what #47 reported.
- The deleted scraping's test cases go with it: the comix cover cases,
  the img[alt] and meta[name] stub branches, and the stale og:image
  fixtures. Export lists are unchanged; nothing cover-specific was
  exported.

The cover route itself is already public and uncredentialed, with the
immutable cache directive and 404-for-unknown covered by the tests that
landed with #59.

userscript/novel-bookmark.user.js

@@ -33,16 +33,11 @@
   // ============================================================
   // Site adapters
   //
-  // Page type + IDs come from URL regex (most stable); title/cover come from
-  // og: meta tags (with the novelfull name= meta as the exception).
+  // Page type + IDs come from URL regex (most stable); the title comes from
+  // og: meta tags or the page's own heading. Covers are never read here: the
+  // backend acquires and serves them itself (ADR-0007).
   // ============================================================
 
-
-  function meta(prop) {
-    const el = document.querySelector('meta[property="' + prop + '"]');
-    return el ? el.getAttribute("content") : null;
-  }
-
   // Chapter lists are read from two places: the page we are standing on, and
   // series pages fetched in the background. Both are reduced to {href, text}
   // pairs so each adapter needs only one rule for picking the latest chapter.
@@ -66,12 +61,6 @@
     return out;
   }
 
-  // novelfull ships no og: tags at all — its cover lives on a name= meta.
-  function metaName(name) {
-    const el = document.querySelector('meta[name="' + name + '"]');
-    return el ? el.getAttribute("content") : null;
-  }
-
   function escapeRe(s) {
     return s.replace(/[.*+?^${}()|[\]\\]/g, "\\$&");
   }
@@ -108,7 +97,6 @@
           // h3.title on a chapter page is the *chapter's* title; the breadcrumb
           // link back to the series page carries the series name.
           title: back ? (back.textContent || "").trim() : "",
-          cover: metaName("image") || "",
           seriesUrl: loc.origin + "/" + m[1] + ".html",
           chapterLabel: "Chapter " + m[2],
           chapterNum: isNaN(num) ? null : num,
@@ -124,7 +112,6 @@
           site: this.site,
           seriesId: m[1],
           title: h3 ? (h3.textContent || "").trim() : "",
-          cover: metaName("image") || "",
           seriesUrl: loc.origin + "/" + m[1] + ".html",
           chapterLabel: null,
           chapterNum: null,
@@ -157,9 +144,6 @@
           seriesId: m[1],
           // The heading is "<Series> Chapter <n>"; drop the suffix.
           title: heading.replace(/\s*Chapter\s+[0-9.]+\s*$/i, "").trim(),
-          // Chapter pages carry no og:image. Empty is safe: every write merges
-          // against the cached row, which keeps the cover the series page gave.
-          cover: "",
           seriesUrl: "https://lightnovelworld.net/novel/" + m[1] + "/",
           chapterLabel: "Chapter " + m[2],
           chapterNum: isNaN(num) ? null : num,
@@ -175,7 +159,6 @@
           site: this.site,
           seriesId: m[1],
           title: h1 ? (h1.textContent || "").trim() : "",
-          cover: meta("og:image") || "",
           seriesUrl: "https://lightnovelworld.net/novel/" + m[1] + "/",
           chapterLabel: null,
           chapterNum: null,
@@ -286,6 +269,10 @@
   async function apiPut(key, obj, { sendStatus = false } = {}) {
     const body = Object.assign({}, obj);
     if (!sendStatus) delete body.status;
+    // Covers belong to the backend, which acquires and serves them itself
+    // (ADR-0007) and ignores an incoming one; a third-party address must never
+    // go back on the wire.
+    delete body.cover;
     const res = await fetch(API_BASE + "/bookmarks/" + encodeURIComponent(key), {
       method: "PUT",
       headers: authHeaders({ "Content-Type": "application/json" }),
@@ -613,7 +600,6 @@
       series_id: p.seriesId,
       title: p.title || (existing && existing.title) || p.seriesId,
       series_url: p.seriesUrl || (existing && existing.series_url) || "",
-      cover: p.cover || (existing && existing.cover) || "",
       last_chapter: p.chapterLabel || (existing && existing.last_chapter) || "",
       last_chapter_num:
         p.chapterNum != null ? p.chapterNum : existing ? existing.last_chapter_num : null,
@@ -636,7 +622,6 @@
       series_id: p.seriesId,
       title: existing.title || p.title || p.seriesId,
       series_url: existing.series_url || p.seriesUrl || "",
-      cover: existing.cover || p.cover || "",
       last_chapter: p.chapterLabel || existing.last_chapter || "",
       last_chapter_num: p.chapterNum != null ? p.chapterNum : existing.last_chapter_num,
       last_chapter_url: p.chapterUrl || "",
@@ -1147,7 +1132,15 @@
     return el("div", { class: "item" + heat }, [
       el("a", { class: "go", href: cont }, [
         b.cover
-          ? el("img", { class: "cover", src: b.cover, loading: "lazy", alt: "" })
+          ? el("img", {
+              class: "cover",
+              src: b.cover,
+              loading: "lazy",
+              alt: "",
+              // A Cover that will not load shows the designed placeholder
+              // rather than the browser's broken-image glyph (#47).
+              onerror: (e) => e.target.replaceWith(el("div", { class: "cover ph" })),
+            })
           : el("div", { class: "cover ph" }),
       ]),
       el("div", { class: "meta" }, [