feat(server/m7-352): AssertEditable + ErrSystemPlaylistReadonly on playlists service

internal/playlists/service.go

@@ -30,10 +30,11 @@ import (
 // Typed errors. The API layer maps each to its wire status code; tests
 // use errors.Is to assert the right path was taken.
 var (
-	ErrNotFound      = errors.New("playlists: playlist not found")
-	ErrForbidden     = errors.New("playlists: forbidden")
-	ErrInvalidInput  = errors.New("playlists: invalid input")
-	ErrTrackNotFound = errors.New("playlists: track not found")
+	ErrNotFound               = errors.New("playlists: playlist not found")
+	ErrForbidden              = errors.New("playlists: forbidden")
+	ErrInvalidInput           = errors.New("playlists: invalid input")
+	ErrTrackNotFound          = errors.New("playlists: track not found")
+	ErrSystemPlaylistReadonly = errors.New("playlists: system playlist is read-only")
 )
 
 // Service wires the sqlc queries + on-disk cover-art directory together.
@@ -292,6 +293,32 @@ func (s *Service) Delete(ctx context.Context, callerID, playlistID pgtype.UUID)
 	return nil
 }
 
+// AssertEditable returns nil if the playlist exists, the caller owns it,
+// AND the playlist is user-created (kind='user'). Returns:
+//   - ErrNotFound — playlist doesn't exist
+//   - ErrForbidden — caller is not the owner
+//   - ErrSystemPlaylistReadonly — playlist exists but kind='system'
+//
+// Edit handlers call this first; on error, the API layer maps the typed
+// error to the appropriate status code.
+func (s *Service) AssertEditable(ctx context.Context, callerID, playlistID pgtype.UUID) error {
+	q := dbq.New(s.pool)
+	p, err := q.GetPlaylist(ctx, playlistID)
+	if err != nil {
+		if errors.Is(err, pgx.ErrNoRows) {
+			return ErrNotFound
+		}
+		return fmt.Errorf("playlists: load for editability check: %w", err)
+	}
+	if !pgtypeUUIDEqual(p.UserID, callerID) {
+		return ErrForbidden
+	}
+	if p.Kind == "system" {
+		return ErrSystemPlaylistReadonly
+	}
+	return nil
+}
+
 // AppendTracks adds the given track ids at the end of the playlist,
 // preserving order. Snapshot fields (title, artist, album, duration_sec)
 // are populated from the tracks/albums/artists join at insert time.