feat(data): add navigation properties to all entity models

src/ClaudeDo.Data/Models/SubtaskEntity.cs

@@ -8,4 +8,7 @@ public sealed class SubtaskEntity
     public bool Completed { get; set; }
     public int OrderNum { get; set; }
     public required DateTime CreatedAt { get; init; }
+
+    // Navigation property
+    public TaskEntity Task { get; set; } = null!;
 }