Revamp API (full text search, relations, filter by license or tag)

[duerrsimon]

To achieve #55 #72 #74 and #73 we need an API behind the frontend.

It could be built on FastAPI to keep a flatfile based managment of the icons.

Each icon has a unique name and a metadata field in the SVG header.
The API server on initialization loads all metadata and relations between icons and then responds to get requests with the list of icons.

[duerrsimon]

On each commit to main the files are synced to the API server and a reindex of the existing data is triggered