feat: add post mention

internal/repository/social_graph_repo.go

@@ -380,6 +380,20 @@ func (r *NotificationRepo) UpsertLike(ctx context.Context, userID, actorID, post
 	return count, emailDue, nil
 }
 
+// CreateMention persists a "you were mentioned" notification linked to the
+// post. Unlike likes there's no aggregation — a mention is a deliberate,
+// low-volume event.
+func (r *NotificationRepo) CreateMention(ctx context.Context, userID, actorID, postID string) error {
+	if _, err := r.pg.Pool.Exec(ctx,
+		`INSERT INTO notifications (user_id, actor_id, kind, post_id)
+		 VALUES ($1, $2, 'post_mention', $3)`,
+		userID, actorID, postID,
+	); err != nil {
+		return fmt.Errorf("create mention notification: %w", err)
+	}
+	return nil
+}
+
 // LikeEmailsEnabled reports whether the user wants aggregated like emails.
 func (r *NotificationRepo) LikeEmailsEnabled(ctx context.Context, userID string) (bool, error) {
 	var enabled bool