First slice of the VAT-consolidation: migration 130 adds a nullable vat_code
snapshot column to quotes + invoices, and the Treuhänder export now prefers the
invoice's snapshotted code over the (mutable) rate→code map, so a historical
invoice's VatCode never changes when codes are re-mapped. Schema-drift guarded;
behaviour-neutral until the editors start writing the snapshot (next slices).
Part of: VAT registry → Settings→Accounting, invoice VAT dropdown, registration/
reclaim toggle.
Luca
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