feat: fix NotifyOnce map key to use composite ArtistID+RGID

internal/notifier/scheduler.go

@@ -43,13 +43,16 @@ func NotifyOnce(ctx context.Context, db *database.DB, sender Sender, cfg config.
 	// Authoritative missing set: external releases with no matching local album.
 	// This is what the Web UI dashboard also shows, so the digest stays
 	// consistent with what the operator sees in the UI.
+	// Using composite key ArtistID|RGID to be defensive - while MusicBrainz RGIDs are
+	// globally unique (UUIDs), this protects against potential data inconsistencies.
 	missing, err := scanner.ScanAll(ctx, db, threshold)
 	if err != nil {
 		return 0, fmt.Errorf("notifier: scan missing releases: %w", err)
 	}
-	missingByRGID := make(map[string]scanner.MissingRelease, len(missing))
+	missingByArtistRGID := make(map[string]scanner.MissingRelease, len(missing))
 	for _, m := range missing {
-		missingByRGID[m.RGID] = m
+		key := m.ArtistID + "|" + m.RGID
+		missingByArtistRGID[key] = m
 	}
 
 	// Restrict to releases not yet notified. A release that is genuinely missing
@@ -61,7 +64,8 @@ func NotifyOnce(ctx context.Context, db *database.DB, sender Sender, cfg config.
 
 	toNotify := make([]scanner.MissingRelease, 0, len(unnotified))
 	for _, r := range unnotified {
-		if m, ok := missingByRGID[r.RGID]; ok {
+		key := r.ArtistID + "|" + r.RGID
+		if m, ok := missingByArtistRGID[key]; ok {
 			toNotify = append(toNotify, m)
 		}
 	}