✨(backend) extract attachment keys from updated content for access

We can't prevent document editors from copy/pasting content to from one
document to another. The problem is that copying content, will copy the
urls pointing to attachments but if we don't do anything, the reader of
the document to which the content is being pasted, may not be allowed to
access the attachment files from the original document.

Using the work from the previous commit, we can grant access to the readers
of the target document by extracting the attachment keys from the content and
adding themto the target document's "attachments" field. Before doing this,
we check that the current user can indeed access the attachment files extracted
from the content and that they are allowed to edit the current document.

src/backend/core/api/serializers.py

@@ -10,7 +10,7 @@ from django.utils.translation import gettext_lazy as _
 import magic
 from rest_framework import exceptions, serializers
 
-from core import enums, models
+from core import enums, models, utils
 from core.services.ai_services import AI_ACTIONS
 from core.services.converter_services import (
     ConversionError,
@@ -268,6 +268,53 @@ class DocumentSerializer(ListDocumentSerializer):
 
         return value
 
+    def save(self, **kwargs):
+        """
+        Process the content field to extract attachment keys and update the document's
+        "attachments" field for access control.
+        """
+        content = self.validated_data.get("content", "")
+        extracted_attachments = set(utils.extract_attachments(content))
+
+        existing_attachments = (
+            set(self.instance.attachments or []) if self.instance else set()
+        )
+        new_attachments = extracted_attachments - existing_attachments
+
+        if new_attachments:
+            attachments_documents = (
+                models.Document.objects.filter(
+                    attachments__overlap=list(new_attachments)
+                )
+                .only("path", "attachments")
+                .order_by("path")
+            )
+
+            user = self.context["request"].user
+            readable_per_se_paths = (
+                models.Document.objects.readable_per_se(user)
+                .order_by("path")
+                .values_list("path", flat=True)
+            )
+            readable_attachments_paths = utils.filter_descendants(
+                [doc.path for doc in attachments_documents],
+                readable_per_se_paths,
+                skip_sorting=True,
+            )
+
+            readable_attachments = set()
+            for document in attachments_documents:
+                if document.path not in readable_attachments_paths:
+                    continue
+                readable_attachments.update(set(document.attachments) & new_attachments)
+
+            # Update attachments with readable keys
+            self.validated_data["attachments"] = list(
+                existing_attachments | readable_attachments
+            )
+
+        return super().save(**kwargs)
+
 
 class ServerCreateDocumentSerializer(serializers.Serializer):
     """