Complete Step 2.2: Add enums for business concepts

Updated ROADMAP.md to mark Step 2.2 ("Enums erstellen") as completed, documenting the creation of `DocumentOperationType` and `ProcessingStatus` enums.

Added `DocumentOperationType` and `ProcessingStatus` enums under the `DocumentOperator.Domain.Models.Enums` namespace to represent document operations and processing statuses, respectively.

Modified `DocumentOperator.Domain.csproj` to remove the `Models\Enums\` folder from the `<ItemGroup>` section, reflecting changes in the inclusion strategy for enums.

DocumentOperator.Domain/Models/Enums/ProcessingStatus.cs

@@ -0,0 +1,9 @@
+namespace DocumentOperator.Domain.Models.Enums;
+
+public enum ProcessingStatus
+{
+    Pending,
+    Processing,
+    Success,
+    Failed
+}