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/migrations/20260409100000_add_submission_status/migration.sql

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+ALTER TABLE "HomeworkSubmission" ADD COLUMN "status" TEXT NOT NULL DEFAULT 'PENDING';
+ALTER TABLE "HomeworkSubmission" ADD COLUMN "statusAt" TIMESTAMP(3);

prisma/migrations/20260409200000_add_feedback_fields/migration.sql

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+ALTER TABLE "HomeworkFeedback" ADD COLUMN "files" JSONB;
+ALTER TABLE "HomeworkFeedback" ADD COLUMN "audioUrl" TEXT;