feat: complete filter centralization pattern and document in CLAUDE.md

internal/scanner/scan.go

@@ -33,6 +33,15 @@ func ScanArtist(ctx context.Context, db *database.DB, artistID string, threshold
 	// the MusicBrainz cache to expire and prune rows on the next re-sync.
 	settings, err := database.GetArtistSettings(db, artistID)
 	if err != nil {
+		// If artist settings don't exist, use empty filter (no filtering)
+		if err == database.ErrArtistNotFound {
+			filter := TypeFilter{
+				IgnoreSingles:      false,
+				IgnoreCompilations: false,
+			}
+			missing := FindMissingReleases(local, external, threshold, filter)
+			return missing, nil
+		}
 		return nil, err
 	}
 	filter := TypeFilter{