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/ActivityRepository.cs

@@ -0,0 +1,43 @@
+using Microsoft.EntityFrameworkCore;
+using Nexus.Api.Data;
+
+namespace Nexus.Api.Repositories;
+
+public sealed class ActivityRepository(NexusDbContext db) : IActivityRepository
+{
+    public Task<List<ActivityEvent>> GetRecentAsync(int take, CancellationToken ct = default)
+        => db.Activity.AsNoTracking().OrderByDescending(x => x.CreatedAt).Take(take).ToListAsync(ct);
+
+    public async Task<(List<ActivityEvent> Items, int TotalCount)> GetPagedAsync(
+        string? type, string? sort, int page, int pageSize, CancellationToken ct = default)
+    {
+        var query = db.Activity.AsNoTracking();
+
+        if (!string.IsNullOrWhiteSpace(type))
+            query = query.Where(x => x.Type == type);
+
+        query = (sort?.ToLowerInvariant()) switch
+        {
+            "oldest" => query.OrderBy(x => x.CreatedAt),
+            _ => query.OrderByDescending(x => x.CreatedAt)
+        };
+
+        var totalCount = await query.CountAsync(ct);
+        var items = await query.Skip((page - 1) * pageSize).Take(pageSize).ToListAsync(ct);
+        return (items, totalCount);
+    }
+
+    public Task<List<ActivityEvent>> GetByAgentAsync(string agentId, int take, CancellationToken ct = default)
+        => db.Activity.AsNoTracking()
+            .Where(x => x.Message.Contains(agentId, StringComparison.OrdinalIgnoreCase) || x.Type == "agent")
+            .OrderByDescending(x => x.CreatedAt)
+            .Take(take)
+            .ToListAsync(ct);
+
+    public async Task<ActivityEvent> AddAsync(ActivityEvent activity, CancellationToken ct = default)
+    {
+        db.Activity.Add(activity);
+        await db.SaveChangesAsync(ct);
+        return activity;
+    }
+}