Report Categories Listing Page

We spoke about exploration navigation for users not being familiar with the taxonomy and provide a listing page with all available report (sub) categories form the tree structure.

API endpoint e.g.

• https://thek.lohro.de/api/v1/recording-categories (complete tree from root nodes)
• https://thek.lohro.de/api/v1/recording-categories/26 (subtree of a given node)

we could also change to output only first level children of a node or root nodes, if requested, but would result in more requests from the frontend. currently LOHRO categories are defined with manageable width and depth.