fix: address fourth code review findings

- Fix FK constraint violation in SyncArtistDiscography: delete
  notifications_sent rows before external_releases to prevent
  constraint failure when re-syncing artists with prior notifications.
- Implement per-artist type filtering: FilterReleaseGroups now accepts
  FilterOptions with IgnoreSingles/IgnoreCompilations flags, read from
  artist_settings table via getArtistFilterOptions.
- Fix inconsistent error wrapping: GetExternalRelease now wraps errors
  with fmt.Errorf like all other functions in the package; updated test
  to use errors.Is for sql.ErrNoRows check.
- Add tests: FilterReleaseGroups ignore singles/compilations,
  SyncArtistDiscography per-artist type filtering, and FK-safe resync.

internal/musicbrainz/api.go

@@ -82,10 +82,17 @@ func (c *MusicBrainzClient) GetArtistReleaseGroups(ctx context.Context, artistMB
 	return allGroups, nil
 }
 
+// FilterOptions holds per-artist type filtering preferences.
+type FilterOptions struct {
+	IgnoreSingles      bool
+	IgnoreCompilations bool
+}
+
 // FilterReleaseGroups applies status and type filtering to a list of release groups.
 // It excludes Bootleg, Promotion, and Pseudo-Release statuses.
-// It includes only Album, Single, EP, and Compilation types.
-func FilterReleaseGroups(groups []ReleaseGroup) []ReleaseGroup {
+// It includes only Album, Single, EP, and Compilation types, unless the type
+// is disabled via FilterOptions.
+func FilterReleaseGroups(groups []ReleaseGroup, opts FilterOptions) []ReleaseGroup {
 	var filtered []ReleaseGroup
 	for _, rg := range groups {
 		if IsStatusExcluded(rg.Status) {
@@ -94,6 +101,12 @@ func FilterReleaseGroups(groups []ReleaseGroup) []ReleaseGroup {
 		if !IsTypeIncluded(rg.Type) {
 			continue
 		}
+		if opts.IgnoreSingles && rg.Type == "Single" {
+			continue
+		}
+		if opts.IgnoreCompilations && rg.Type == "Compilation" {
+			continue
+		}
 		filtered = append(filtered, rg)
 	}
 	return filtered