Database refactoring: cron expression vs recurringtype enum

[f3l1x98]

Currently it is only inteded to be possible to create recurring expenses/incomes using the yearly, monthly or weekly enum. If that does not change it would be easier to replace the stored cron expression with the enum value (or at least add the enum with additional custom value).
This would provide the following benefits:

• Backend filtering for recurring type
• Simplify recurring UI (no longer necessary to convert enum to cron or cron to enum)

[f3l1x98]

Due to backend scheduled execution still requiring cron expression, the best solution would probably to add the enum and hide the cron expression from the frontend.