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/security/JwtAuthenticationFilter.java

@@ -45,7 +45,7 @@ public class JwtAuthenticationFilter extends OncePerRequestFilter {
 
         final String authHeader = request.getHeader("Authorization");
         final String jwt;
-        final String userEmail;
+        final String username;
 
         if (!StringUtils.hasText(authHeader) || !authHeader.startsWith("Bearer ")) {
             filterChain.doFilter(request, response);
@@ -62,15 +62,16 @@ public class JwtAuthenticationFilter extends OncePerRequestFilter {
         }
 
         try {
-            userEmail = jwtService.extractUsername(jwt);
+            username = jwtService.extractUsername(jwt); // This is actually the `username`, not email
         } catch (Exception e) {
             response.setStatus(HttpServletResponse.SC_UNAUTHORIZED);
             response.getWriter().write("Invalid JWT token");
             return;
         }
 
-        if (userEmail != null && SecurityContextHolder.getContext().getAuthentication() == null) {
-            User user = userRepository.findByEmail(userEmail).orElse(null);
+        if (username != null && SecurityContextHolder.getContext().getAuthentication() == null) {
+            // ❗ Use username to find the user
+            User user = userRepository.findByUsername(username).orElse(null);
 
             if (user != null && jwtService.isTokenValid(jwt, user)) {
                 UsernamePasswordAuthenticationToken authToken = new UsernamePasswordAuthenticationToken(