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.go

@@ -41,6 +41,16 @@ func extractRecordingMBID(m tag.Metadata) string {
 	return cleanMBID(mbz.Extract(m).Get(mbz.Recording))
 }
 
+// extractReleaseGroupMBID reads the MusicBrainz *release-group* ID —
+// Picard's musicbrainz_releasegroupid, surfaced by dhowden/tag as
+// mbz.ReleaseGroup. mbz.Album (what albums.mbid stores) is the release: one
+// edition. Lidarr names albums by release group, so re-acquisition and
+// request completion key on this one (#5241). Separate from extractMBIDs for
+// the same reason extractRecordingMBID is.
+func extractReleaseGroupMBID(m tag.Metadata) string {
+	return cleanMBID(mbz.Extract(m).Get(mbz.ReleaseGroup))
+}
+
 func cleanMBID(s string) string {
 	if s == "" {
 		return ""