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/TaskRepository.cs

@@ -0,0 +1,53 @@
+using Microsoft.EntityFrameworkCore;
+using Nexus.Api.Data;
+
+namespace Nexus.Api.Repositories;
+
+public sealed class TaskRepository(NexusDbContext db) : ITaskRepository
+{
+    public Task<List<WorkTask>> GetAllAsync(CancellationToken ct = default)
+        => db.Tasks.AsNoTracking().OrderByDescending(x => x.UpdatedAt).ToListAsync(ct);
+
+    public ValueTask<WorkTask?> GetByIdAsync(Guid id, CancellationToken ct = default)
+        => db.Tasks.FindAsync([id], ct);
+
+    public Task<List<WorkTask>> GetPendingApprovalAsync(CancellationToken ct = default)
+    {
+        var threshold = DateTimeOffset.UtcNow.AddHours(-1);
+        return db.Tasks.AsNoTracking()
+            .Where(x => x.State == TaskStateHelper.ToStateString(TaskState.InProgress) && x.UpdatedAt <= threshold)
+            .OrderByDescending(x => x.UpdatedAt)
+            .ToListAsync(ct);
+    }
+
+    public async Task<WorkTask> AddAsync(WorkTask task, CancellationToken ct = default)
+    {
+        db.Tasks.Add(task);
+        await db.SaveChangesAsync(ct);
+        return task;
+    }
+
+    public async Task UpdateAsync(WorkTask task, CancellationToken ct = default)
+    {
+        task.UpdatedAt = DateTimeOffset.UtcNow;
+        await db.SaveChangesAsync(ct);
+    }
+
+    public async Task DeleteAsync(WorkTask task, CancellationToken ct = default)
+    {
+        db.Tasks.Remove(task);
+        await db.SaveChangesAsync(ct);
+    }
+
+    public Task<int> CountAsync(CancellationToken ct = default)
+        => db.Tasks.CountAsync(ct);
+
+    public Task<int> CountByStateAsync(string state, CancellationToken ct = default)
+        => db.Tasks.CountAsync(x => x.State == state, ct);
+
+    public Task<WorkTask?> GetLastBlockedAsync(CancellationToken ct = default)
+        => db.Tasks.AsNoTracking()
+            .Where(x => x.State == TaskStateHelper.ToStateString(TaskState.Blocked))
+            .OrderByDescending(x => x.UpdatedAt)
+            .FirstOrDefaultAsync(ct);
+}