feat(invoices): Unify billing and commercial Invoice entities
- Create unified Invoice entity in invoices module with all fields:
- Commercial fields: salesOrderId, purchaseOrderId, partnerId
- SaaS fields: subscriptionId, periodStart, periodEnd
- Context discriminator: invoiceContext ('commercial' | 'saas')
- Mark billing-usage/invoice.entity.ts as deprecated, re-export from invoices
- Update billing-usage/services/invoices.service.ts for unified entity
- Add missing InvoiceStatus values (validated, cancelled, voided)
- Export types from both modules for backward compatibility
Note: financial/invoice.entity.ts remains separate (different schema)
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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