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/Repositories/ProjectRepository.cs

@@ -0,0 +1,35 @@
+using Microsoft.EntityFrameworkCore;
+using Nexus.Api.Data;
+
+namespace Nexus.Api.Repositories;
+
+public sealed class ProjectRepository(NexusDbContext db) : IProjectRepository
+{
+    public Task<List<Project>> GetAllAsync(CancellationToken ct = default)
+        => db.Projects.AsNoTracking().OrderByDescending(x => x.UpdatedAt).ToListAsync(ct);
+
+    public ValueTask<Project?> GetByIdAsync(Guid id, CancellationToken ct = default)
+        => db.Projects.FindAsync([id], ct);
+
+    public async Task<Project> AddAsync(Project project, CancellationToken ct = default)
+    {
+        db.Projects.Add(project);
+        await db.SaveChangesAsync(ct);
+        return project;
+    }
+
+    public async Task UpdateAsync(Project project, CancellationToken ct = default)
+    {
+        project.UpdatedAt = DateTimeOffset.UtcNow;
+        await db.SaveChangesAsync(ct);
+    }
+
+    public async Task DeleteAsync(Project project, CancellationToken ct = default)
+    {
+        db.Projects.Remove(project);
+        await db.SaveChangesAsync(ct);
+    }
+
+    public Task<bool> HasTasksAsync(Guid projectId, CancellationToken ct = default)
+        => db.Tasks.AnyAsync(t => t.ProjectId == projectId, ct);
+}