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/mbids_test.go

@@ -163,3 +163,40 @@ func TestCleanMBID_OnlyNULReturnsEmpty(t *testing.T) {
 		t.Errorf("got %q, want empty", got)
 	}
 }
+
+// #5241: the release-group id is read beside the release id, from each
+// format's own key, and the two are never confused.
+func TestExtractReleaseGroupMBID(t *testing.T) {
+	cases := []struct {
+		name string
+		meta stubMeta
+	}{
+		{"vorbis", stubMeta{format: tag.VORBIS, raw: map[string]interface{}{
+			"musicbrainz_albumid":        "release-1",
+			"musicbrainz_releasegroupid": "group-1",
+		}}},
+		{"id3v2", stubMeta{format: tag.ID3v2_4, raw: map[string]interface{}{
+			"TXXX":   &tag.Comm{Description: "MusicBrainz Album Id", Text: "release-1"},
+			"TXXX_0": &tag.Comm{Description: "MusicBrainz Release Group Id", Text: "group-1\x00"},
+		}}},
+		{"mp4", stubMeta{format: tag.MP4, raw: map[string]interface{}{
+			"MusicBrainz Album Id":         "release-1",
+			"MusicBrainz Release Group Id": "group-1",
+		}}},
+	}
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			if got := extractReleaseGroupMBID(c.meta); got != "group-1" {
+				t.Errorf("release group = %q, want group-1", got)
+			}
+			if album, _ := extractMBIDs(c.meta); album != "release-1" {
+				t.Errorf("album (release) = %q, want release-1", album)
+			}
+		})
+	}
+
+	none := stubMeta{format: tag.VORBIS, raw: map[string]interface{}{"musicbrainz_albumid": "release-1"}}
+	if got := extractReleaseGroupMBID(none); got != "" {
+		t.Errorf("untagged release group = %q, want empty", got)
+	}
+}