✨(backend) use langfuse to monitor AI actions

We want to monitor AI actions. For this we choose to use langfuse. As
this usage is optional, we load langfuse sdk only if settings are
configured. Also, the openai client from langfuse is a dropin
replacement of openai client, so we only have to change how openai is
imported.

src/backend/pyproject.toml

@@ -48,6 +48,7 @@ dependencies = [
     "factory_boy==3.3.3",
     "gunicorn==23.0.0",
     "jsonschema==4.25.1",
+    "langfuse==3.11.2",
     "lxml==6.0.2",
     "markdown==3.10",
     "mozilla-django-oidc==5.0.2",