✨(backend) add documents/all endpoint with descendants

[ChristopherSpelt]

Purpose

This PR adds a new /api/v1.0/documents/all/ endpoint that returns all accessible documents including their complete hierarchies (children, grandchildren, etc.), unlike the existing list endpoint which only returns top-level documents.

Resolves #1497

External contributions

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[berrydenhartog]

Thanks @ChristopherSpelt for implementing this. If this gets merged we can attach our Dashboard application and search on titles. Maybe later we can even add advanced search with integrated OpenSearch 🥇

@lunika We will also add a similair PR for Drive soon