feat(library): store each album's MusicBrainz release-group id (M483 #5242)

albums.mbid is the release id (Picard's musicbrainz_albumid, one edition).
Lidarr names albums by release group, so re-acquisition and request
completion need that id too (#5241).

Migration 0070 adds albums.release_group_mbid (nullable, non-unique index:
several releases share a group). The scanner reads musicbrainz_releasegroupid
through extractReleaseGroupMBID, writes it on insert and heals it onto
existing rows when NULL. tagReadVersion goes to 3 so the next scan fills
it for the library already indexed, bound by tag reads (no ffprobe, no
decode).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/library/scanner_test.go

@@ -308,3 +308,80 @@ func TestScanner_AdoptsMovedFile_Integration(t *testing.T) {
 		t.Errorf("tracks = %d, want 8 — a rename must not add a row", total)
 	}
 }
+
+// TestScanner_ReleaseGroupMBID_Integration is #5241's scan half: a new album
+// takes its release-group id from the tag, and an album indexed before this
+// read existed gets it on the next scan, through the tagReadVersion bump,
+// without its file changing.
+func TestScanner_ReleaseGroupMBID_Integration(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping scanner integration in -short mode")
+	}
+	dsn := os.Getenv("MINSTREL_TEST_DATABASE_URL")
+	if dsn == "" {
+		t.Skip("MINSTREL_TEST_DATABASE_URL not set")
+	}
+	ctx := context.Background()
+	logger := slog.New(slog.NewTextHandler(io.Discard, nil))
+
+	if err := db.Migrate(dsn, logger); err != nil {
+		t.Fatalf("migrate: %v", err)
+	}
+	pool, err := pgxpool.New(ctx, dsn)
+	if err != nil {
+		t.Fatalf("pool: %v", err)
+	}
+	t.Cleanup(pool.Close)
+	if _, err := pool.Exec(ctx, "TRUNCATE tracks, albums, artists RESTART IDENTITY CASCADE"); err != nil {
+		t.Fatalf("truncate: %v", err)
+	}
+
+	const (
+		groupA = "aaaaaaaa-1111-2222-3333-444444444444"
+		groupB = "bbbbbbbb-1111-2222-3333-444444444444"
+	)
+	root := t.TempDir()
+	writeTestMP3(t, filepath.Join(root, "rg/A/01.mp3"), map[string]string{
+		"TIT2": "One", "TPE1": "Artist RG", "TALB": "Album A",
+		"TXXX": "MusicBrainz Release Group Id\x00" + groupA,
+	})
+	pathB := filepath.Join(root, "rg/B/01.mp3")
+	writeTestMP3(t, pathB, map[string]string{
+		"TIT2": "Two", "TPE1": "Artist RG", "TALB": "Album B",
+		"TXXX": "MusicBrainz Release Group Id\x00" + groupB,
+	})
+
+	scanner := New(pool, logger, []string{root}, nil)
+	if _, err := scanner.Scan(ctx, nil); err != nil {
+		t.Fatalf("first scan: %v", err)
+	}
+	groupOf := func(title string) *string {
+		t.Helper()
+		var rg *string
+		if err := pool.QueryRow(ctx, "SELECT release_group_mbid FROM albums WHERE title = $1", title).Scan(&rg); err != nil {
+			t.Fatalf("album %q: %v", title, err)
+		}
+		return rg
+	}
+	if rg := groupOf("Album A"); rg == nil || *rg != groupA {
+		t.Fatalf("new album release group = %v, want %s", rg, groupA)
+	}
+
+	// Album B as a library indexed under tag-read version 2 left it: no
+	// release group, file untouched since.
+	if _, err := pool.Exec(ctx, "UPDATE albums SET release_group_mbid = NULL WHERE title = 'Album B'"); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := pool.Exec(ctx, "UPDATE tracks SET tag_read_version = 2 WHERE file_path = $1", pathB); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := scanner.Scan(ctx, nil); err != nil {
+		t.Fatalf("second scan: %v", err)
+	}
+	if rg := groupOf("Album B"); rg == nil || *rg != groupB {
+		t.Errorf("existing album release group after rescan = %v, want %s", rg, groupB)
+	}
+	if rg := groupOf("Album A"); rg == nil || *rg != groupA {
+		t.Errorf("album A release group changed to %v", rg)
+	}
+}