feat: redesign detail page with stage modal and milestone APIs

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

backend/prisma/schema.prisma

@@ -95,17 +95,20 @@ enum Priority {
 // ─── 업무 상세 / 진행 기록 ────────────────────────────────────
 
 model TaskDetail {
-  id        String   @id @default(cuid())
-  taskId    String
-  content   String
-  updatedBy String
-  createdAt DateTime @default(now())
-  updatedAt DateTime @updatedAt
+  id          String   @id @default(cuid())
+  taskId      String
+  milestoneId String?
+  content     String
+  updatedBy   String
+  createdAt   DateTime @default(now())
+  updatedAt   DateTime @updatedAt
 
-  task   Task @relation(fields: [taskId], references: [id], onDelete: Cascade)
-  author User @relation(fields: [updatedBy], references: [id])
+  task      Task       @relation(fields: [taskId], references: [id], onDelete: Cascade)
+  milestone Milestone? @relation(fields: [milestoneId], references: [id], onDelete: Cascade)
+  author    User       @relation(fields: [updatedBy], references: [id])
 
   @@index([taskId])
+  @@index([milestoneId])
   @@map("task_details")
 }
 
@@ -132,6 +135,7 @@ model KpiMetric {
 model File {
   id           String   @id @default(cuid())
   taskId       String
+  milestoneId  String?
   filename     String   // 저장된 파일명 (UUID)
   originalName String   // 원본 파일명
   mimetype     String
@@ -140,10 +144,12 @@ model File {
   uploadedBy   String
   createdAt    DateTime @default(now())
 
-  task     Task @relation(fields: [taskId], references: [id], onDelete: Cascade)
-  uploader User @relation(fields: [uploadedBy], references: [id])
+  task      Task       @relation(fields: [taskId], references: [id], onDelete: Cascade)
+  milestone Milestone? @relation(fields: [milestoneId], references: [id], onDelete: SetNull)
+  uploader  User       @relation(fields: [uploadedBy], references: [id])
 
   @@index([taskId])
+  @@index([milestoneId])
   @@map("files")
 }
 
@@ -154,13 +160,17 @@ model Milestone {
   taskId      String
   title       String
   description String?
+  startDate   DateTime?
   dueDate     DateTime?
+  links       String?   // JSON: [{ "label": string, "url": string }]
   completedAt DateTime?
   order       Int       @default(0)
   createdAt   DateTime  @default(now())
   updatedAt   DateTime  @updatedAt
 
-  task Task @relation(fields: [taskId], references: [id], onDelete: Cascade)
+  task    Task         @relation(fields: [taskId], references: [id], onDelete: Cascade)
+  details TaskDetail[]
+  files   File[]
 
   @@index([taskId])
   @@map("milestones")