MINOR: Delete flashcards

Description

Flashcards can be deleted - this includes new frontend code and a new backend component

[x] Does backend/database/database_config.py have the type of production?
[x] Does frontend/src/api/config.js have the correct server url, reading from the deployed instance at http://dolphinflashcards.com/api/?
[x] If not merging development to main, is the MR title prefixed with MAJOR, MINOR or PATCH?
[x] Has backend/__init__.py been updated with the relevant version, following the semantic versioning standard
[x] Has CHANGELOG.md been updated to explain the new version?