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/prisma/migrations/20260714185225_add_unmatched_album/migration.sql

@@ -0,0 +1,15 @@
+-- CreateTable
+CREATE TABLE "UnmatchedAlbum" (
+    "id" TEXT NOT NULL,
+    "artist" TEXT NOT NULL,
+    "album" TEXT NOT NULL,
+    "path" TEXT NOT NULL,
+    "reason" TEXT NOT NULL,
+    "scanId" TEXT NOT NULL,
+    "scannedAt" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT "UnmatchedAlbum_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateIndex
+CREATE UNIQUE INDEX "UnmatchedAlbum_path_key" ON "UnmatchedAlbum"("path");