Add findPage helper to JooqBaseEntityRepository

[Rattenkrieg]

Summary

JooqBaseEntityRepository (the base for CreatableEntity repos) didn't expose a paging helper, so consumers had to reach for raw queryExecutor.executeSelect and reimplement the seek/paging plumbing locally. This adds findPage mirroring the one on JooqBaseModelRepository, reusing the same PagingStrategy contract and BasicPagedList wiring — and extracts the shared body so the two repositories don't carry mirrored code.

Scope

• New PagingExecution.kt with internal executeFindPage(...) carrying the seek/select/map body shared by both repositories.
• New findPage method on JooqBaseEntityRepository delegating to executeFindPage, with the same generics and contract as the model-repository version.
• JooqBaseModelRepository.findPage refactored to delegate to executeFindPage (was allRecords(... .page(...)) plus a private SelectConditionStep<R>.page extension — both gone).
• JooqBaseModelRepository's queryExecutor constructor parameter is now stored as a private field so it can be passed to the shared helper. Constructor signature unchanged.
• No behaviour changes to existing callers.