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/Requests.cs

@@ -0,0 +1,48 @@
+namespace Nexus.Api.DTOs;
+
+public sealed record CreateProjectRequest(string Name, string? Description);
+public sealed record CreateTaskRequest(string Title, string? Priority, Guid? ProjectId);
+public sealed record UpdateTaskStateRequest(string State);
+public sealed record ChatRequest(string Message, string? ConversationId, string? AgentId);
+
+public sealed record UpdateProjectRequest(string? Name, string? Description, string? Status);
+public sealed record UpdateTaskRequest(string? Title, string? Priority, Guid? ProjectId);
+
+public sealed record AgentCommandRequest(string Message);
+
+public sealed record SaveConfigRequest(string Content);
+
+public sealed record AgentListResponse(
+    string Id,
+    string Name,
+    string Role,
+    string Model,
+    string Status,
+    DateTimeOffset? LastSeen,
+    string? Workspace,
+    string? Description
+);
+
+public sealed record AgentDetailResponse(
+    string Id,
+    string Name,
+    string Role,
+    string Model,
+    string Status,
+    DateTimeOffset? LastSeen,
+    string? Workspace,
+    string? AgentDir,
+    string? Description,
+    IReadOnlyList<string>? SubAgents,
+    string? IdentityName
+);
+
+public sealed record AgentCommandResponse(
+    string Runtime,
+    string AgentId,
+    string ConversationId,
+    string Content
+);
+
+public sealed record CronJobEntry(string Id, string Name, string Schedule, string LastRun, string NextRun, string Status);
+public sealed record UpcomingCronEntry(string Id, string Name, string NextRun, string Schedule);