feat(server/m7-379): heal existing artist+album mbid on rescan

internal/db/queries/artists.sql

@@ -74,3 +74,12 @@ WHERE a.id = $1
   AND NOT EXISTS (SELECT 1 FROM albums al WHERE al.artist_id = a.id)
   AND NOT EXISTS (SELECT 1 FROM tracks t WHERE t.artist_id = a.id)
 RETURNING a.id;
+
+-- name: SetArtistMbidIfNull :exec
+-- M7 #379: heal MBID on existing artist rows during rescan. Idempotent —
+-- only updates when the existing mbid is NULL, so concurrent scans don't
+-- overwrite an existing value with a stale one.
+UPDATE artists
+   SET mbid = $2,
+       updated_at = now()
+ WHERE id = $1 AND mbid IS NULL;