Database Refactor

Description

Seperated concrete database implementation classes from api methods to seperate classes in database/handlers
Added much more unit tests, and made the pipeline run all unit tests
Added methods to help with unit tests like /api/get-user-stats

Switched to Firebase Firestore instead of Firebase Database to allow referencing data, not just embedding - leading to greater scaliability
Changed frontend to interact with the new database

Deprecated /api/calculate-card-stats - this will now be done by the client to improve speed
Deprecated /api/get-weekly-xp and /api/get-total-xp - the frontend calculates them at the same time, so it makes sense to use /api/get-user-stats to pull this data instead

[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?