Nebenbuch Implementation — Master Task List

Phase 1: Foundation (Documentation Only) ✅

[x] Schema docs: change_events with nullable FKs in all 4 schemas (incl. passenger_profile, reseller, allotment, tour_offering, ticket, crew_duty_log, boarding_event)
[x] NestJS docs: tax-engine.md, period-lock.md, storno-workflow.md, datev-integration.md
[x] ADR adr-002-financial-ledger-strategy.md updated
[x] Implementation plan finalized with all user feedback

[ ] Hasura Migration: change_events tables (4 schemas, nullable FKs, CHECK constraint)
[ ] Hasura Migration: commerce.ledger_period_locks, invoice_cancellations, reconciliation_*
[ ] NestJS: CommerceModule (period-lock, storno counter-document, audit handlers per entity incl. offering-audit)
[ ] NestJS: BackofficeModule (costing-sheet, config, passenger DSGVO audit)
[ ] NestJS: OperationsModule (expense GOBD, crew-duty COMPLIANCE, boarding audit)
[ ] Register all modules in app.module.ts

[ ] Shared tax types: enum, Valibot schemas, margin/vat calculators (dynamic tax rate)
[ ] NestJS: TaxResolverService (Backoffice — determines if FREMD exists → strategy)
[ ] NestJS: TaxCalculatorService (Commerce — calculates § 25 Abs. 5 specific values). Drittland taxRate=0 edge case)