Adds customer_accounts.skonto_disabled (migration 112) so a customer
that negotiated "no early-payment discount" can be flagged once instead
of ticking the per-invoice toggle on every invoice. resolveSkontoPercent
ForInvoice and the PDF render context both honour it, extending the
resolution chain to customer → invoice → snapshot → quote → global.
Checkbox added to the customer detail Billing card (en + de).