✨(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/models.py

@@ -662,6 +662,7 @@ class Document(MP_Node, BaseModel):
             "invite_owner": is_owner,
             "move": is_owner_or_admin and not self.ancestors_deleted_at,
             "partial_update": can_update,
+            "restore": is_owner,
             "retrieve": can_get,
             "media_auth": can_get,
             "update": can_update,
@@ -753,6 +754,52 @@ class Document(MP_Node, BaseModel):
             ancestors_deleted_at=self.ancestors_deleted_at
         )
 
+    @transaction.atomic
+    def restore(self):
+        """Cancelling a soft delete with checks."""
+        # This should not happen
+        if self.deleted_at is None:
+            raise ValidationError({"deleted_at": [_("This document is not deleted.")]})
+
+        if self.deleted_at < get_trashbin_cutoff():
+            raise ValidationError(
+                {
+                    "deleted_at": [
+                        _(
+                            "This document was permanently deleted and cannot be restored."
+                        )
+                    ]
+                }
+            )
+
+        # Restore the current document
+        self.deleted_at = None
+
+        # Calculate the minimum `deleted_at` among all ancestors
+        ancestors_deleted_at = (
+            self.get_ancestors()
+            .filter(deleted_at__isnull=False)
+            .values_list("deleted_at", flat=True)
+        )
+        self.ancestors_deleted_at = min(ancestors_deleted_at, default=None)
+        self.save()
+
+        # Update descendants excluding those who were deleted prior to the deletion of the
+        # current document (the ancestor_deleted_at date for those should already by good)
+        # The number of deleted descendants should not be too big so we can handcraft a union
+        # clause for them:
+        deleted_descendants_paths = (
+            self.get_descendants()
+            .filter(deleted_at__isnull=False)
+            .values_list("path", flat=True)
+        )
+        exclude_condition = models.Q(
+            *(models.Q(path__startswith=path) for path in deleted_descendants_paths)
+        )
+        self.get_descendants().exclude(exclude_condition).update(
+            ancestors_deleted_at=self.ancestors_deleted_at
+        )
+
 
 class LinkTrace(BaseModel):
     """