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/db/queries/albums.sql

@@ -1,6 +1,6 @@
 -- name: UpsertAlbum :one
-INSERT INTO albums (title, sort_title, artist_id, release_date, mbid, cover_art_path)
-VALUES ($1, $2, $3, $4, $5, $6)
+INSERT INTO albums (title, sort_title, artist_id, release_date, mbid, cover_art_path, release_group_mbid)
+VALUES ($1, $2, $3, $4, $5, $6, $7)
 ON CONFLICT (mbid) WHERE mbid IS NOT NULL
 DO UPDATE SET
     title = EXCLUDED.title,
@@ -8,6 +8,7 @@ DO UPDATE SET
     artist_id = EXCLUDED.artist_id,
     release_date = EXCLUDED.release_date,
     cover_art_path = EXCLUDED.cover_art_path,
+    release_group_mbid = COALESCE(EXCLUDED.release_group_mbid, albums.release_group_mbid),
     updated_at = now()
 RETURNING *;
 
@@ -142,6 +143,15 @@ UPDATE albums
        updated_at = now()
  WHERE id = $1 AND mbid IS NULL;
 
+-- name: SetAlbumReleaseGroupMbidIfNull :exec
+-- #5241: heal the release-group id on an existing album row, from a rescan's
+-- tags or from the re-acquisition sweeper's MusicBrainz lookup. Only fills a
+-- NULL, so a tag never gets overwritten by a later guess or vice versa.
+UPDATE albums
+   SET release_group_mbid = $2,
+       updated_at = now()
+ WHERE id = $1 AND release_group_mbid IS NULL;
+
 -- name: ListAlbumsMissingMbidWithTrack :many
 -- One-shot MBID backfill: returns each album where mbid IS NULL alongside
 -- one of its tracks' file_path so the worker can re-read tags. LIMIT