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/Entities.cs

@@ -0,0 +1,92 @@
+namespace Nexus.Api.Data;
+
+public enum OperationalStatus
+{
+    Online,
+    Degraded,
+    Offline,
+    Unknown
+}
+
+/// <summary>
+/// Strongly-typed task lifecycle states.
+/// String values (e.g. "In progress") are preserved for API compatibility
+/// via <see cref="TaskStateHelper"/>; the WorkTask entity continues to store
+/// state as a string in the database.
+/// </summary>
+public enum TaskState
+{
+    Backlog,
+    InProgress,
+    Blocked,
+    Done
+}
+
+public static class TaskStateHelper
+{
+    private static readonly Dictionary<TaskState, string> StateToString = new()
+    {
+        [TaskState.Backlog] = "Backlog",
+        [TaskState.InProgress] = "In progress",
+        [TaskState.Blocked] = "Blocked",
+        [TaskState.Done] = "Done"
+    };
+
+    private static readonly Dictionary<string, TaskState> StringToState = new(StringComparer.OrdinalIgnoreCase)
+    {
+        ["Backlog"] = TaskState.Backlog,
+        ["In progress"] = TaskState.InProgress,
+        ["Blocked"] = TaskState.Blocked,
+        ["Done"] = TaskState.Done
+    };
+
+    /// <summary>Valid task-state string values for API validation.</summary>
+    public static readonly string[] AllStates = ["Backlog", "In progress", "Blocked", "Done"];
+
+    /// <summary>Convert a TaskState enum to its API string representation.</summary>
+    public static string ToStateString(this TaskState state) => StateToString[state];
+
+    /// <summary>Parse a string to TaskState; defaults to Backlog for unrecognized input.</summary>
+    public static TaskState ToTaskState(this string state) =>
+        StringToState.TryGetValue(state, out var result) ? result : TaskState.Backlog;
+
+    /// <summary>Returns true if the string is a recognized task state (case-insensitive).</summary>
+    public static bool IsValidState(string? state) =>
+        !string.IsNullOrWhiteSpace(state) && StringToState.ContainsKey(state);
+
+    public static bool IsInProgressOrBlocked(string? state) =>
+        string.Equals(state, "In progress", StringComparison.OrdinalIgnoreCase)
+        || string.Equals(state, "Blocked", StringComparison.OrdinalIgnoreCase);
+
+    public static bool IsDoneOrBacklog(string? state) =>
+        string.Equals(state, "Done", StringComparison.OrdinalIgnoreCase)
+        || string.Equals(state, "Backlog", StringComparison.OrdinalIgnoreCase);
+}
+
+public sealed class Project
+{
+    public Guid Id { get; init; } = Guid.NewGuid();
+    public required string Name { get; set; }
+    public string Description { get; set; } = string.Empty;
+    public int Progress { get; set; }
+    public OperationalStatus Status { get; set; } = OperationalStatus.Unknown;
+    public DateTimeOffset UpdatedAt { get; set; } = DateTimeOffset.UtcNow;
+}
+
+public sealed class WorkTask
+{
+    public Guid Id { get; init; } = Guid.NewGuid();
+    public required string Title { get; set; }
+    public string State { get; set; } = "Backlog";
+    public string Priority { get; set; } = "Normal";
+    public Guid? ProjectId { get; set; }
+    public DateTimeOffset UpdatedAt { get; set; } = DateTimeOffset.UtcNow;
+}
+
+public sealed class ActivityEvent
+{
+    public long Id { get; init; }
+    public required string Type { get; set; }
+    public required string Message { get; set; }
+    public DateTimeOffset CreatedAt { get; set; } = DateTimeOffset.UtcNow;
+}