feat(core): add IRsaKeyService interface

NEW/src/JdeScoping.Core/Interfaces/IRsaKeyService.cs

@@ -0,0 +1,19 @@
+namespace JdeScoping.Core.Interfaces;
+
+/// <summary>
+/// RSA key management for login encryption.
+/// </summary>
+public interface IRsaKeyService
+{
+    /// <summary>
+    /// Gets the server's public key in PEM format.
+    /// </summary>
+    string GetPublicKeyPem();
+
+    /// <summary>
+    /// Decrypts RSA-OAEP encrypted data.
+    /// </summary>
+    /// <param name="ciphertext">Encrypted bytes</param>
+    /// <returns>Decrypted plaintext bytes</returns>
+    byte[] Decrypt(byte[] ciphertext);
+}