fix: feedback author display, sidebar scroll, stage sort by start date

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

backend/prisma/schema.prisma

@@ -99,6 +99,7 @@ model TaskDetail {
   taskId      String
   milestoneId String?
   content     String
+  authorName  String?  // 피드백 작성자 표시명 (자유 입력)
   updatedBy   String
   createdAt   DateTime @default(now())
   updatedAt   DateTime @updatedAt