Backend rework

[solevis]

Major backend code update:

• [x] Use enum for type in Record model
• [x] Use enum for type in DailySale model
• [x] Remove internal caching system and replace it by Flask-Caching
• [x] Split PixyShip class into multiple services
• [x] Fix majority of ruff warnings
• [x] Check typing with mypy
• [x] Add missing docstrings
• [x] Remove CHANGELOG

I've decided to stop using a version system for PixyShip. In future, I'll be using GitHub flow for development, which means that there will only be the main branch and PRs.