Rich Question Content Upgrade

This release adds rich question content, image uploads, and server-side HTML sanitization.

Production Upgrade

Rebuild or pull a newly built app image because multer and sanitize-html were added.
Run ./backup.sh before deployment.
Create the host upload directory before first deployment: mkdir -p uploads.
docker-compose.yml mounts ./uploads:/app/uploads; include this directory in backups.
Deploy with docker compose pull app or docker compose build --no-cache app, then docker compose up -d app.
Verify with docker compose logs -f app --since 1m and /api/health.
Smoke test by creating a question with a code block, link, and uploaded image, then checking both editor preview and the student exam page.
Rollback can use the old image. The DB columns are additive, but rich questions should not be edited in the old UI after rollback.