✨(backend) add API endpoint action to restore a soft deleted document

Only owners can see and restore deleted documents. They can only do
it during the grace period before the document is considered hard
deleted and hidden from everybody on the API.

src/backend/core/api/viewsets.py

@@ -714,6 +714,22 @@ class DocumentViewSet(
             {"message": "Document moved successfully."}, status=status.HTTP_200_OK
         )
 
+    @drf.decorators.action(
+        detail=True,
+        methods=["post"],
+    )
+    def restore(self, request, *args, **kwargs):
+        """
+        Restore a soft-deleted document if it was deleted less than x days ago.
+        """
+        document = self.get_object()
+        document.restore()
+
+        return drf_response.Response(
+            {"detail": "Document has been successfully restored."},
+            status=status.HTTP_200_OK,
+        )
+
     @drf.decorators.action(
         detail=True,
         methods=["get", "post"],