fix: production save errors and import HR dashboard data

Resolve invalid task creator IDs, fix API routing and file uploads on Vercel, and replace dummy seed data with HR_Dashboard import.

backend/src/lib/resolveUser.ts

@@ -1,6 +1,22 @@
 import { prisma } from './prisma';
 import { AppError } from '../middleware/errorHandler';
 
+/** 업무 생성 시 사용할 creatorId (클라이언트의 잘못된 값 무시) */
+export async function resolveCreatorId(requested?: string): Promise<string> {
+  if (requested?.trim() && requested !== 'system') {
+    const user = await prisma.user.findUnique({ where: { id: requested.trim() } });
+    if (user) return user.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.');
+}
+
 /** 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({