Add secure file upload and download with per-user AES encryption

- FileController encrypts uploads using AES-GCM with salt and IV.
- Downloads are decrypted on-the-fly using user-supplied password.
- File metadata (salt, IV, username, path) stored in DB.

src/main/java/com/skycrate/backend/skycrateBackend/entity/FileMetadata.java

@@ -0,0 +1,23 @@
+package com.skycrate.backend.skycrateBackend.entity;
+
+import jakarta.persistence.*;
+
+@Entity
+public class FileMetadata {
+
+    @Id
+    @GeneratedValue(strategy = GenerationType.IDENTITY)
+    private Long id;
+
+    private String filePath;
+
+    private String username;
+
+    @Lob
+    private byte[] salt;
+
+    @Lob
+    private byte[] iv;
+
+    // Getters and Setters
+}