Refactor file upload/download with service layer and secure encryption

- Replaced direct encryption logic in FileController with FileService delegation
- Added JWT-based username extraction in file operations
- Updated FileMetadata entity to include `uploadedAt` field and removed redundant getters/setters
- Refactored EncryptionUtil:
  - Switched to AES-CBC with PBKDF2 key derivation
  - Removed RSA-based encryption logic
  - Added salt and IV generation helpers
- Changed JwtAuthenticationFilter to fetch user by username (not email)
- Renamed method in FileMetadataRepository to match new parameter order

FILE UPLOAD NOW WORKS! TESTED USING CURL.

src/main/java/com/skycrate/backend/skycrateBackend/services/JwtService.java

@@ -63,4 +63,4 @@ public class JwtService {
     public String generateToken(User user) {
         return generateToken((UserDetails) user);
     }
-}
+}