feat: redesign detail page with stage modal and milestone APIs

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

backend/src/routes/files.ts

@@ -16,15 +16,26 @@ router.post('/upload/:taskId', upload.single('file'), async (req, res, next) =>
     const task = await prisma.task.findUnique({ where: { id: taskId } });
     if (!task) throw new AppError(404, '업무를 찾을 수 없습니다.');
 
+    const body = req.body as Record<string, string>;
+    const milestoneId = body.milestoneId?.trim() || null;
+
+    if (milestoneId) {
+      const milestone = await prisma.milestone.findFirst({
+        where: { id: milestoneId, taskId },
+      });
+      if (!milestone) throw new AppError(404, '단계를 찾을 수 없습니다.');
+    }
+
     const fileRecord = await prisma.file.create({
       data: {
         taskId,
+        milestoneId,
         filename: req.file.filename,
         originalName: req.file.originalname,
         mimetype: req.file.mimetype,
         size: req.file.size,
         path: req.file.path,
-        uploadedBy: (req.body as Record<string, string>).uploadedBy ?? 'system',
+        uploadedBy: body.uploadedBy ?? 'system',
       },
     });