feat(library): on-demand integrity check

GET /api/library/integrity stats every album folder on disk (via the /music
mount) and flags: missing folders, folders with no audio, zero-byte/
unreadable (truncated) files, and on-disk track count drift vs the names
captured at import/scan. Metadata-only (readdir + stat), so it runs only when
the user clicks "Check now" from a new Integrity section on /library; each
flagged album opens its modal to fix or delete. SectionHeader.note widened to
ReactNode to host the button. web 178 tests.

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

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

@@ -33,11 +33,14 @@ 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 };
+type IntegrityIssue = { id: string; artist: string; album: string; issues: string[] };
 
 export function LibraryClient() {
   const [albums, setAlbums] = useState<Album[]>([]);
   const [unmatched, setUnmatched] = useState<Unmatched[]>([]);
   const [duplicates, setDuplicates] = useState<DupItem[][]>([]);
+  const [integrity, setIntegrity] = useState<{ checked: number; issues: IntegrityIssue[] } | null>(null);
+  const [checking, setChecking] = useState(false);
   const [q, setQ] = useState("");
   const [sort, setSort] = useState<SortKey>("added");
   const [open, setOpen] = useState<Album | null>(null);
@@ -112,6 +115,20 @@ export function LibraryClient() {
     refresh();
   }, []);
 
+  async function checkIntegrity() {
+    setChecking(true);
+    try {
+      const res = await fetch("/api/library/integrity").catch(() => null);
+      if (res?.ok) {
+        setIntegrity(await res.json());
+      } else {
+        toast("Couldn't run the integrity check");
+      }
+    } finally {
+      setChecking(false);
+    }
+  }
+
   async function dismissUnmatched(u: Unmatched) {
     const res = await fetch(`/api/library/unmatched?id=${u.id}`, { method: "DELETE" }).catch(() => null);
     if (res?.ok) {
@@ -175,6 +192,51 @@ export function LibraryClient() {
         </div>
       ) : null}
 
+      {albums.length > 0 ? (
+        <>
+          <SectionHeader
+            title="Integrity"
+            note={
+              <button className="btn sm ghost" onClick={checkIntegrity} disabled={checking}>
+                {checking ? "Checking…" : "Check now"}
+              </button>
+            }
+          />
+          {integrity ? (
+            integrity.issues.length === 0 ? (
+              <p className="muted-note">Checked {integrity.checked} albums — no problems found.</p>
+            ) : (
+              <ul className="list">
+                {integrity.issues.map((it) => {
+                  const album = albums.find((a) => a.id === it.id);
+                  return (
+                    <li key={it.id} className="list-row">
+                      <div className="main">
+                        {album ? (
+                          <button className="linkish rtitle" onClick={() => show(album)}>
+                            {it.artist} — {it.album}
+                          </button>
+                        ) : (
+                          <div className="rtitle">{it.artist} — {it.album}</div>
+                        )}
+                        <div className="rmeta">
+                          <span className="score">{it.issues.join(" · ")}</span>
+                        </div>
+                      </div>
+                    </li>
+                  );
+                })}
+              </ul>
+            )
+          ) : (
+            <p className="muted-note">
+              Check every album folder on disk for missing folders, empty/truncated files, and
+              track-count drift.
+            </p>
+          )}
+        </>
+      ) : null}
+
       {duplicates.length > 0 ? (
         <>
           <SectionHeader title="Possible duplicates" note={`${duplicates.length}`} />