✨(backend) add comment viewset

This commit add the CRUD part to manage comment lifeycle. Permissions
are relying on the Document and Comment abilities. Comment viewset
depends on the Document route and is added to the
document_related_router. Dedicated serializer and permission are
created.

src/backend/core/api/serializers.py

@@ -891,3 +891,47 @@ class MoveDocumentSerializer(serializers.Serializer):
         choices=enums.MoveNodePositionChoices.choices,
         default=enums.MoveNodePositionChoices.LAST_CHILD,
     )
+
+
+class CommentSerializer(serializers.ModelSerializer):
+    """Serialize comments."""
+
+    user = UserLightSerializer(read_only=True)
+    abilities = serializers.SerializerMethodField(read_only=True)
+
+    class Meta:
+        model = models.Comment
+        fields = [
+            "id",
+            "content",
+            "created_at",
+            "updated_at",
+            "user",
+            "document",
+            "abilities",
+        ]
+        read_only_fields = [
+            "id",
+            "created_at",
+            "updated_at",
+            "user",
+            "document",
+            "abilities",
+        ]
+
+    def get_abilities(self, comment) -> dict:
+        """Return abilities of the logged-in user on the instance."""
+        request = self.context.get("request")
+        if request:
+            return comment.get_abilities(request.user)
+        return {}
+
+    def validate(self, attrs):
+        """Validate invitation data."""
+        request = self.context.get("request")
+        user = getattr(request, "user", None)
+
+        attrs["document_id"] = self.context["resource_id"]
+        attrs["user_id"] = user.id if user else None
+
+        return attrs