refactor: Clean Architecture mit Repository Pattern, Controllern und DTOs

- 15 Controller-Klassen ersetzen Minimal APIs in Program.cs
- Repository Pattern mit Interfaces + Implementierungen (Project, Task, Activity, User)
- AuthService verwendet jetzt IUserRepository statt direktem DbContext-Zugriff
- SecurityHeadersMiddleware als eigenständige Middleware-Klasse
- PathSecurityHelper als gemeinsamer Helper für Pfadvalidierung
- DTOs in eigenem Namespace Nexus.Api.DTOs
- EF-Entities in Nexus.Api.Data (vorher Nexus.Api.Domain)
- Program.cs auf DI-Registrierung + Middleware reduziert
- Alle 43 Endpoints unverändert erhalten
- Build + 3/3 Tests erfolgreich

backend/DTOs/AuthRequests.cs

@@ -0,0 +1,42 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace Nexus.Api.DTOs;
+
+public sealed record LoginRequest
+{
+    [Required, EmailAddress, MaxLength(120)]
+    public string Email { get; init; } = string.Empty;
+
+    [Required, MinLength(10), MaxLength(200)]
+    public string Password { get; init; } = string.Empty;
+}
+
+public sealed record AuthResponse
+{
+    public string AccessToken { get; init; } = string.Empty;
+    public DateTimeOffset ExpiresAt { get; init; }
+    public UserInfo User { get; init; } = new();
+}
+
+public sealed record UserInfo
+{
+    public Guid Id { get; init; }
+    public string Email { get; init; } = string.Empty;
+    public string DisplayName { get; init; } = string.Empty;
+    public string Role { get; init; } = string.Empty;
+}
+
+public sealed record UpdateProfileRequest
+{
+    [MaxLength(100)]
+    public string? DisplayName { get; init; }
+}
+
+public sealed record ChangePasswordRequest
+{
+    [Required, MinLength(10), MaxLength(200)]
+    public string CurrentPassword { get; init; } = string.Empty;
+
+    [Required, MinLength(10), MaxLength(200)]
+    public string NewPassword { get; init; } = string.Empty;
+}