fix: stage save errors and add milestone progress field

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

backend/src/routes/files.ts

@@ -2,6 +2,7 @@ import { Router } from 'express';
 import path from 'path';
 import fs from 'fs';
 import { prisma } from '../lib/prisma';
+import { resolveTaskActorId } from '../lib/resolveUser';
 import { upload } from '../middleware/upload';
 import { AppError } from '../middleware/errorHandler';
 
@@ -26,6 +27,8 @@ router.post('/upload/:taskId', upload.single('file'), async (req, res, next) =>
       if (!milestone) throw new AppError(404, '단계를 찾을 수 없습니다.');
     }
 
+    const uploadedBy = await resolveTaskActorId(taskId);
+
     const fileRecord = await prisma.file.create({
       data: {
         taskId,
@@ -35,7 +38,7 @@ router.post('/upload/:taskId', upload.single('file'), async (req, res, next) =>
         mimetype: req.file.mimetype,
         size: req.file.size,
         path: req.file.path,
-        uploadedBy: body.uploadedBy ?? 'system',
+        uploadedBy,
       },
     });