Important refinement:
for authentication, authorization, tenant/account lookup, and user access control, you must reuse the existing MongoDB database `duxiter` and the current collections already used by the Node.js backend.

Do not introduce new auth collections, user collections, or tenant collections unless absolutely unavoidable.
First inspect the codebase and map the existing schema.
Then adapt the FastAPI backend to that schema.

Compatibility with the existing Mongo data model is more important than idealized redesign.