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/Data/Identity.cs

@@ -0,0 +1,45 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace Nexus.Api.Data;
+
+public class NexusUser
+{
+    public Guid Id { get; set; } = Guid.NewGuid();
+
+    [MaxLength(120)]
+    public string Email { get; set; } = string.Empty;
+
+    [MaxLength(120)]
+    public string NormalizedEmail { get; set; } = string.Empty;
+
+    [MaxLength(100)]
+    public string DisplayName { get; set; } = string.Empty;
+
+    public string PasswordHash { get; set; } = string.Empty;
+
+    public string Role { get; set; } = "owner";
+
+    public DateTimeOffset CreatedAt { get; set; } = DateTimeOffset.UtcNow;
+
+    public DateTimeOffset UpdatedAt { get; set; } = DateTimeOffset.UtcNow;
+
+    public DateTimeOffset? LastLoginAt { get; set; }
+
+    public ICollection<RefreshToken> RefreshTokens { get; set; } = new List<RefreshToken>();
+}
+
+public class RefreshToken
+{
+    public Guid Id { get; set; } = Guid.NewGuid();
+    public Guid UserId { get; set; }
+    [MaxLength(64)]
+    public string TokenHash { get; set; } = string.Empty;
+    public Guid FamilyId { get; set; } = Guid.NewGuid();
+    public DateTimeOffset ExpiresAt { get; set; }
+    public DateTimeOffset CreatedAt { get; set; } = DateTimeOffset.UtcNow;
+    public DateTimeOffset? RevokedAt { get; set; }
+    [MaxLength(64)]
+    public string? ReplacedByTokenHash { get; set; }
+    public Guid ConcurrencyStamp { get; set; } = Guid.NewGuid();
+    public NexusUser User { get; set; } = null!;
+}