feat: add S3 signed URL generation for image uploads

- Implemented a new API route for generating signed URLs for S3 uploads in `src/app/api/storage/sign/route.ts`.
- Added input validation using Zod to ensure the request body contains an array of keys.
- Integrated AWS SDK to create signed URLs for specified keys with a 7-day expiration.
- Created an S3 client in `src/backend/s3.client.ts` to handle S3 interactions with environment-specific configurations.

Author: kingRayhan