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/api/library/unmatched/route.test.ts

@@ -0,0 +1,33 @@
+import { describe, it, expect } from "vitest";
+import { prisma } from "@/lib/db";
+import { GET, DELETE } from "./route";
+
+describe("library unmatched API", () => {
+  it("lists unmatched albums ordered by artist/album", async () => {
+    await prisma.unmatchedAlbum.createMany({
+      data: [
+        { artist: "Zed", album: "Late", path: "/music/Zed/Late", reason: "no MusicBrainz match", scanId: "s1" },
+        { artist: "Ann", album: "Early", path: "/music/Ann/Early", reason: "unreadable: boom", scanId: "s1" },
+      ],
+    });
+    const res = await GET();
+    expect(res.status).toBe(200);
+    const { unmatched } = await res.json();
+    expect(unmatched.map((u: { artist: string }) => u.artist)).toEqual(["Ann", "Zed"]);
+    expect(unmatched[0]).toMatchObject({ album: "Early", reason: "unreadable: boom" });
+  });
+
+  it("dismisses one entry by id", async () => {
+    const row = await prisma.unmatchedAlbum.create({
+      data: { artist: "X", album: "Y", path: "/music/X/Y", reason: "no MusicBrainz match", scanId: "s1" },
+    });
+    const res = await DELETE(new Request(`http://t/api/library/unmatched?id=${row.id}`, { method: "DELETE" }));
+    expect(res.status).toBe(200);
+    expect(await prisma.unmatchedAlbum.count()).toBe(0);
+  });
+
+  it("404s dismissing an unknown id", async () => {
+    const res = await DELETE(new Request("http://t/api/library/unmatched?id=nope", { method: "DELETE" }));
+    expect(res.status).toBe(404);
+  });
+});

web/src/app/api/library/unmatched/route.ts

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+import { prisma } from "@/lib/db";
+
+// GET /api/library/unmatched — album folders the scan couldn't resolve to a MusicBrainz
+// release (no match, or an unreadable file). Surfaced on /library so they aren't silently lost.
+export async function GET() {
+  const rows = await prisma.unmatchedAlbum.findMany({ orderBy: [{ artist: "asc" }, { album: "asc" }] });
+  return Response.json({
+    unmatched: rows.map((r) => ({
+      id: r.id,
+      artist: r.artist,
+      album: r.album,
+      path: r.path,
+      reason: r.reason,
+      scannedAt: r.scannedAt,
+    })),
+  });
+}
+
+// DELETE /api/library/unmatched?id= — dismiss one entry (the user has handled it, or doesn't
+// care). Does not touch files on disk; a later re-scan re-surfaces it if still unresolved.
+export async function DELETE(request: Request) {
+  const id = new URL(request.url).searchParams.get("id");
+  if (!id) return Response.json({ error: "id is required" }, { status: 400 });
+  const deleted = await prisma.unmatchedAlbum.deleteMany({ where: { id } });
+  if (deleted.count === 0) return Response.json({ error: "not found" }, { status: 404 });
+  return Response.json({ ok: true });
+}