feat(library): surface possible duplicate albums

GET /api/library/duplicates groups library items that look like the same
release held more than once — sharing a MusicBrainz release-group, or the
same artist + edition-normalized title (stripEditionQualifiers), unioned so
transitive matches collapse into one group. The (artist,album) unique
constraint rules out exact dupes, so these near-dupes are what's worth
flagging.

/library shows a "Possible duplicates" section; each entry opens the album
modal to edit or delete. web 175 tests.

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

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

@@ -32,10 +32,12 @@ const SORTS: { key: SortKey; label: string; cmp: (a: Album, b: Album) => number
 ];
 
 type Unmatched = { id: string; artist: string; album: string; path: string; reason: string };
+type DupItem = { id: string; artist: string; album: string; format: string; qualityClass: number; year: string | null };
 
 export function LibraryClient() {
   const [albums, setAlbums] = useState<Album[]>([]);
   const [unmatched, setUnmatched] = useState<Unmatched[]>([]);
+  const [duplicates, setDuplicates] = useState<DupItem[][]>([]);
   const [q, setQ] = useState("");
   const [sort, setSort] = useState<SortKey>("added");
   const [open, setOpen] = useState<Album | null>(null);
@@ -102,6 +104,9 @@ export function LibraryClient() {
     fetch("/api/library/unmatched")
       .then((r) => r.json())
       .then((d) => setUnmatched(d.unmatched ?? []));
+    fetch("/api/library/duplicates")
+      .then((r) => r.json())
+      .then((d) => setDuplicates(d.duplicates ?? []));
   }
   useEffect(() => {
     refresh();
@@ -170,6 +175,43 @@ export function LibraryClient() {
         </div>
       ) : null}
 
+      {duplicates.length > 0 ? (
+        <>
+          <SectionHeader title="Possible duplicates" note={`${duplicates.length}`} />
+          <p className="muted-note">
+            Albums that look like the same release held more than once (same MusicBrainz release,
+            or the same title ignoring edition wording). Open one to edit or delete it.
+          </p>
+          {duplicates.map((group, gi) => (
+            <ul className="list" key={gi} style={{ marginBottom: 10 }}>
+              {group.map((d) => {
+                const album = albums.find((a) => a.id === d.id);
+                return (
+                  <li key={d.id} className="list-row">
+                    <div className="main">
+                      {album ? (
+                        <button className="linkish rtitle" onClick={() => show(album)}>
+                          {d.artist} — {d.album}
+                        </button>
+                      ) : (
+                        <div className="rtitle">{d.artist} — {d.album}</div>
+                      )}
+                      <div className="rmeta">
+                        <span className="score">
+                          {d.format}
+                          {d.qualityClass >= 3 ? " · hi-res" : ""}
+                          {d.year ? ` · ${d.year}` : ""}
+                        </span>
+                      </div>
+                    </div>
+                  </li>
+                );
+              })}
+            </ul>
+          ))}
+        </>
+      ) : null}
+
       {unmatched.length > 0 ? (
         <>
           <SectionHeader title="Couldn’t match" note={`${unmatched.length}`} />