feat(core): add shared auth models for encrypted login

NEW/src/JdeScoping.Core/Models/Auth/EncryptedLoginRequest.cs

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+namespace JdeScoping.Core.Models.Auth;
+
+/// <summary>
+/// Encrypted login payload sent from client to API.
+/// </summary>
+/// <param name="EncryptedData">Base64-encoded RSA-encrypted JSON of LoginModel</param>
+public record EncryptedLoginRequest(string EncryptedData);

NEW/src/JdeScoping.Core/Models/Auth/LoginModel.cs

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+using System.ComponentModel.DataAnnotations;
+
+namespace JdeScoping.Core.Models.Auth;
+
+/// <summary>
+/// Login credentials model shared by Client and API.
+/// </summary>
+public class LoginModel
+{
+    [Required(ErrorMessage = "Username is required")]
+    public string Username { get; set; } = string.Empty;
+
+    [Required(ErrorMessage = "Password is required")]
+    public string Password { get; set; } = string.Empty;
+}

NEW/src/JdeScoping.Core/Models/Auth/LoginResultModel.cs

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+using JdeScoping.Core.Models;
+
+namespace JdeScoping.Core.Models.Auth;
+
+/// <summary>
+/// Result returned from login API endpoint.
+/// </summary>
+/// <param name="Success">Whether authentication succeeded</param>
+/// <param name="ErrorMessage">Error message if failed</param>
+/// <param name="User">User info if successful</param>
+public record LoginResultModel(
+    bool Success,
+    string? ErrorMessage,
+    UserInfo? User);

NEW/src/JdeScoping.Core/Models/Auth/PublicKeyResponse.cs

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+namespace JdeScoping.Core.Models.Auth;
+
+/// <summary>
+/// Server's RSA public key for client-side encryption.
+/// </summary>
+/// <param name="PublicKeyPem">PEM-encoded RSA public key</param>
+public record PublicKeyResponse(string PublicKeyPem);