Remove any usages of Flask

[puehringer]

visyn_core depends on Flask for a few endpoints (like xlsx, dbmigration (can be removed), mainapp (can be removed), ...). It should be very easy to migrate them to FastAPI (or remove them altogether if not used anymore). Then, we can make Flask optional and only import it when we run init_legacy_app (only used by some legacy apps which then need to install flask themselves). This also means that all apps using Flask need to put it in their own requirements.txt (or use tdp_core, which will also get it).