✨(api) allow forcing ID when creating a document via API endpoint

We need to be able to force the ID when creating a document via
the API endpoint. This is usefull for documents that are created
offline as synchronization is achieved by replaying stacked requests.

We do it via the serializer, making sure that we don't override an
existing document.

src/backend/core/api/serializers.py

@@ -163,6 +163,29 @@ class DocumentSerializer(BaseResourceSerializer):
             "updated_at",
         ]
 
+    def get_fields(self):
+        """Dynamically make `id` read-only on PUT requests but writable on POST requests."""
+        fields = super().get_fields()
+
+        request = self.context.get("request")
+        if request and request.method == "POST":
+            fields["id"].read_only = False
+
+        return fields
+
+    def validate_id(self, value):
+        """Ensure the provided ID does not already exist when creating a new document."""
+        request = self.context.get("request")
+
+        # Only check this on POST (creation)
+        if request and request.method == "POST":
+            if models.Document.objects.filter(id=value).exists():
+                raise serializers.ValidationError(
+                    "A document with this ID already exists. You cannot override it."
+                )
+
+        return value
+
 
 class LinkDocumentSerializer(BaseResourceSerializer):
     """