Improve /recent/help and /recent/demo

[JonasNorlinder]

The endpoints /recent/help and /recent/demo are not doing first data filtration at DB-level, which causes substantial performance regression the more requests a teacher have handled.

See e.g. https://github.com/bubify/bubify-backend/blob/main/src/main/java/com/uu/au/controllers/GodController.java#L682-L698.

See

bubify-backend/src/main/java/com/uu/au/controllers/DemonstrationController.java

Line 339 in f226236

public List<Demonstration> pending() {

how data can be filtered at DB-level instead of application level.