fix: stage save errors and add milestone progress field

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/src/lib/resolveUser.ts

@@ -0,0 +1,23 @@
+import { prisma } from './prisma';
+import { AppError } from '../middleware/errorHandler';
+
+/** task 작성자 또는 관리자 등 유효한 user id 반환 (FK 오류 방지) */
+export async function resolveTaskActorId(taskId: string): Promise<string> {
+  const task = await prisma.task.findUnique({
+    where: { id: taskId },
+    select: { creatorId: true },
+  });
+
+  if (task?.creatorId) {
+    const creator = await prisma.user.findUnique({ where: { id: task.creatorId } });
+    if (creator) return creator.id;
+  }
+
+  const admin = await prisma.user.findFirst({ where: { role: 'ADMIN' }, select: { id: true } });
+  if (admin) return admin.id;
+
+  const anyUser = await prisma.user.findFirst({ select: { id: true } });
+  if (anyUser) return anyUser.id;
+
+  throw new AppError(500, '사용자를 찾을 수 없습니다. 관리자 계정을 먼저 생성해 주세요.');
+}