feat: core invoicing — InvoiceService, API routes, API tests
Phase 2:
- InvoiceService: manual create, create from transaction, create from
repair ticket, payment application (single + FIFO), void, write-off,
send, list, export CSV, AR aging report, overdue marking
- Invoice API routes: full CRUD, payment application, void, write-off,
account invoices/balance, CSV export, AR aging, outstanding accounts
- 14 API tests covering invoice CRUD, payment workflow, validation,
void, write-off, export, aging report
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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