Makes the built-in dunning flow a faithful replacement for the hardcoded
reminder ladder instead of a disabled representation:
- queue_payment_check action delegates to invoiceService.queuePaymentCheckEmail,
so the proven confirm + reminder_level + Mahngebühr state machine
(recordPaymentCheckAction) stays the single source of truth — the workflow
only decides WHEN the payment-check email (the gate) fires.
- runScheduledTasks now SKIPS the hardcoded reminder batches when workflows is
on AND the invoice_dunning built-in is enabled, so the two never double-send.
- The built-in graph is re-authored to the delegation model (wait→due, grace,
loop: check-paid → payment-check → wait-gap), dropping the redundant gate +
generic reminder emails. A SEED_VERSION re-seeds the disabled, never-activated
built-in on boot but never touches an enabled/edited one.
Tests: delegation graph shape, re-seed-when-stale, enabled-protection (9 engine
+ 8 route = 17 passing).