chore: added software architecture doc / refactor migrations

[SeverinAlexB]

Me playing around with AI

• AI generated software architecture doc which is pretty good.
• Refactored migrations with AI. Currently migrations are all defined in the sql folder. This violates the Directed Acyclical Graph though aka sql can access module above. This PR moves the migrations to the individual modules.

[86667]

Migration refactor all good 👍

The doc could be trimmed down a bit, the shorter it is the more chance that devs actually read it.

Parts that are probably not needed to be documented:

• Many of the code examples
• Directory structure diagram
• Router factory pattern
• listing of background tasks and code example of syncer
• Error handling

All of these details can be found by looking at the code

Things which the doc should be used to explain:

• Architecture overview and its characteristics
• How resource sharing works and rules for adding new modules
• Layer diagrams and layer responsibilities
• Dependency rules
• checklist for new modules

[SeverinAlexB]

Closing because I want to keep refining the approach.