Add homework review workflow: statuses, audio, file attachments, tabs

- HomeworkSubmission: add status (PENDING/REVIEWING/APPROVED/REJECTED) + statusAt
- HomeworkFeedback: add files (Json) + audioUrl fields
- Curator detail page: meta table, content tabs, feedback history with audio/files
- FeedbackForm: file upload, audio recorder (Web Audio API + S3), action buttons
- AudioRecorder component: record → preview → upload to S3
- ContentTabs: toggle between homework description and lesson content (TipTap read-only)
- Homework list: 4-color status badges with proper filtering
- API routes: /api/curator/upload and /api/curator/audio-upload

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

prisma/schema.prisma

@@ -258,12 +258,14 @@ model Homework {
 }
 
 model HomeworkSubmission {
-  id         String   @id @default(cuid())
-  homeworkId String
-  userId     String
-  text       String?
-  files      Json?
-  submittedAt DateTime @default(now())
+  id          String    @id @default(cuid())
+  homeworkId  String
+  userId      String
+  text        String?
+  files       Json?
+  status      String    @default("PENDING") // PENDING | REVIEWING | APPROVED | REJECTED
+  statusAt    DateTime?
+  submittedAt DateTime  @default(now())
 
   homework  Homework           @relation(fields: [homeworkId], references: [id], onDelete: Cascade)
   user      User               @relation(fields: [userId], references: [id], onDelete: Cascade)
@@ -275,6 +277,8 @@ model HomeworkFeedback {
   submissionId String
   curatorId    String
   text         String
+  files        Json?    // [{name, url, size}]
+  audioUrl     String?
   createdAt    DateTime @default(now())
 
   submission HomeworkSubmission @relation(fields: [submissionId], references: [id], onDelete: Cascade)