Immutable, gap-free invoicing ships — GoBD compliance by construction

An invoice is only "sent" once — after that, its title, recipient, line items and amounts are locked at the database level. There is no admin override, no quiet edit. A mistake after issue is fixed the only way tax law allows: a credit note that references the original.

Invoice numbers are assigned in a single gap-free sequence per workspace per year, generated at send time rather than at draft time — the classic reason invoice runs end up with holes in them. Legal entities, VAT ID validation and system tax rates (19% / 7% / Reverse Charge, plus custom rates) round out the finance foundation this ships on.

This is the compliance baseline every later finance feature builds on — self-billing, XRechnung export and the payment ledger all assume it.