security: add file type and size validation for presigned URL endpoint

- Add strict validation for contentType in presign endpoint
- Add mandatory file size validation (required, > 0, <= 20MB)
- Extract reusable constants (MAX_FILE_SIZE, MAX_FILES, MAX_TOTAL_FILE_SIZE)
- Create validateFileSize function for code reuse
- Fix security issue 1.3: presigned URL missing file type and size restrictions

Author: Rabithua