♻️(back) extract ancestor deleted_at directly from db in restore method

In the restore method, all the ancestors with a deleted_at date set are
extracted from the database and then the oldest value is extracted using
the min python function. This usage of min can be removed by sorting
directly the deleted_at at the databse level and then fetching the first
one. It's faster and easier to maintain.

src/backend/core/models.py

@@ -794,9 +794,11 @@ class Document(MP_Node, BaseModel):
         ancestors_deleted_at = (
             self.get_ancestors()
             .filter(deleted_at__isnull=False)
+            .order_by("deleted_at")
             .values_list("deleted_at", flat=True)
+            .first()
         )
-        self.ancestors_deleted_at = min(ancestors_deleted_at, default=None)
+        self.ancestors_deleted_at = ancestors_deleted_at
         self.save()
 
         # Update descendants excluding those who were deleted prior to the deletion of the