Clean up Table Schema

[samuelstroschein]

Context

For simplicity, we refrained from correctly defining table types as kysely describes in https://kysely.dev/docs/getting-started#types.

• leads to type problems with generated columns (LIX-95)
• defined created_at as potentially undefined even though its always defined but inserts don't have to provide the type

Proposal

Define the database schema correctly like https://kysely.dev/docs/getting-started#types shows.