✨(backend) add soft delete to documents and refactor db queryset

Now that we have introduced a document tree structure, it is not
possible to allow deleting documents anymore as it impacts the whole
subtree below the deleted document and the consequences are too big.

We introduce soft delete in order to give a second thought to the
document's owner (who is the only one to be allowed to delete a
document). After a document is soft deleted, the owner can still
see it in the trashbin (/api/v1.0/documents/trashbin).
After a grace period (30 days be default) the document disappears
from the trashbin and can't be restored anymore. Note that even
then it is still kept in database. Cleaning the database to erase
deleted documents after the grace period can be done as a maintenance
script.

src/backend/core/api/permissions.py

@@ -2,10 +2,11 @@
 
 from django.core import exceptions
 from django.db.models import Q
+from django.http import Http404
 
 from rest_framework import permissions
 
-from core.models import DocumentAccess, RoleChoices
+from core.models import DocumentAccess, RoleChoices, get_trashbin_cutoff
 
 ACTION_FOR_METHOD_TO_PERMISSION = {
     "versions_detail": {"DELETE": "versions_destroy", "GET": "versions_retrieve"},
@@ -110,3 +111,26 @@ class AccessPermission(permissions.BasePermission):
         except KeyError:
             pass
         return abilities.get(action, False)
+
+
+class DocumentAccessPermission(AccessPermission):
+    """Subclass to handle soft deletion specificities."""
+
+    def has_object_permission(self, request, view, obj):
+        """
+        Return a 404 on deleted documents
+        - for which the trashbin cutoff is past
+        - for which the current user is not owner of the document or one of its ancestors
+        """
+        if (
+            deleted_at := obj.ancestors_deleted_at
+        ) and deleted_at < get_trashbin_cutoff():
+            raise Http404
+
+        # Compute permission first to ensure the "user_roles" attribute is set
+        has_permission = super().has_object_permission(request, view, obj)
+
+        if obj.ancestors_deleted_at and not RoleChoices.OWNER in obj.user_roles:
+            raise Http404
+
+        return has_permission