feat(library): surface albums the scan couldn't match

The library scan silently skipped album folders with no MusicBrainz match
(or an unreadable file). Persist them so they're visible and fixable.

- New UnmatchedAlbum model (migration add_unmatched_album), keyed by path.
- scan_chunk records a skipped album as unmatched (reason "no MusicBrainz
  match" or "unreadable: <err>"), clears the row when an album later
  resolves, and prunes stale rows (a prior scan's, or a removed folder)
  when a scan completes.
- GET /api/library/unmatched lists them; DELETE ?id= dismisses one.
- /library shows a "Couldn't match" section (path + reason + Dismiss).

Worker + web tests added.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/app/library/library-client.tsx

@@ -31,8 +31,11 @@ const SORTS: { key: SortKey; label: string; cmp: (a: Album, b: Album) => number
   { key: "artist", label: "Artist A–Z", cmp: (a, b) => a.artist.localeCompare(b.artist) || (a.year ?? "").localeCompare(b.year ?? "") },
 ];
 
+type Unmatched = { id: string; artist: string; album: string; path: string; reason: string };
+
 export function LibraryClient() {
   const [albums, setAlbums] = useState<Album[]>([]);
+  const [unmatched, setUnmatched] = useState<Unmatched[]>([]);
   const [q, setQ] = useState("");
   const [sort, setSort] = useState<SortKey>("added");
   const [open, setOpen] = useState<Album | null>(null);
@@ -60,11 +63,23 @@ export function LibraryClient() {
     fetch("/api/library")
       .then((r) => r.json())
       .then((d) => setAlbums(d.albums ?? []));
+    fetch("/api/library/unmatched")
+      .then((r) => r.json())
+      .then((d) => setUnmatched(d.unmatched ?? []));
   }
   useEffect(() => {
     refresh();
   }, []);
 
+  async function dismissUnmatched(u: Unmatched) {
+    const res = await fetch(`/api/library/unmatched?id=${u.id}`, { method: "DELETE" }).catch(() => null);
+    if (res?.ok) {
+      setUnmatched((list) => list.filter((x) => x.id !== u.id));
+    } else {
+      toast("Couldn't dismiss");
+    }
+  }
+
   const shown = useMemo(() => {
     const needle = q.trim().toLowerCase();
     const filtered = needle
@@ -119,6 +134,38 @@ export function LibraryClient() {
         </div>
       ) : null}
 
+      {unmatched.length > 0 ? (
+        <>
+          <SectionHeader title="Couldn’t match" note={`${unmatched.length}`} />
+          <p className="muted-note">
+            Album folders on disk the scan couldn’t resolve to a MusicBrainz release. Fix the
+            folder name (Artist / Album (Year)) or re-add them manually, then re-scan. Dismiss to
+            hide an entry.
+          </p>
+          <ul className="list">
+            {unmatched.map((u) => (
+              <li key={u.id} className="list-row">
+                <div className="main">
+                  <div className="rtitle">
+                    {u.artist ? `${u.artist} — ` : ""}
+                    {u.album}
+                  </div>
+                  <div className="rmeta">
+                    <span className="dim">{u.path}</span> <span className="dot">·</span>{" "}
+                    <span className="score">{u.reason}</span>
+                  </div>
+                </div>
+                <div className="actions">
+                  <button className="btn sm ghost" onClick={() => dismissUnmatched(u)}>
+                    Dismiss
+                  </button>
+                </div>
+              </li>
+            ))}
+          </ul>
+        </>
+      ) : null}
+
       {open ? (
         <AlbumModal
           open