♻️(backend) remove content from list serializer and introduce excerpt

Including the content field in the list view is not efficient as we need
to query the object storage to retrieve it. We want to display an excerpt
of the content on the list view so we should store it in database. We
let the frontend compute it and save it for us in the new "excerpt" field
because we are not supposed to have access to the content (E2EE feature coming)

src/backend/core/api/serializers.py

@@ -152,11 +152,11 @@ class ListDocumentSerializer(BaseResourceSerializer):
         model = models.Document
         fields = [
             "id",
-            "content",
             "abilities",
             "created_at",
             "creator",
             "depth",
+            "excerpt",
             "is_favorite",
             "link_role",
             "link_reach",
@@ -172,6 +172,7 @@ class ListDocumentSerializer(BaseResourceSerializer):
             "created_at",
             "creator",
             "depth",
+            "excerpt",
             "is_favorite",
             "link_role",
             "link_reach",
@@ -196,6 +197,7 @@ class DocumentSerializer(ListDocumentSerializer):
             "created_at",
             "creator",
             "depth",
+            "excerpt",
             "is_favorite",
             "link_role",
             "link_reach",