feat(billing): PDF generation EN16931 + Romanian law compliance

[mostlyvirtual]

Summary

The PDF generation for invoices and proformas (via ReportLab in pdf_generators.py) needs EN16931 and Romanian legal compliance upgrades. Currently it generates basic coordinate-based PDFs without embedded XML, full Romanian fields, or structured tax breakdowns.

Current State

The PDF generator (RomanianDocumentPDFGenerator in services/platform/apps/billing/pdf_generators.py) uses ReportLab with manual coordinate-based text placement. It includes basic company info, client info, items table, and totals.

Missing for EN16931 Compliance

• PDF/A-3 format with embedded UBL XML — EN16931 requires the XML invoice to be embedded in the PDF for e-invoicing interoperability
• Invoice series concept (serie + numar) — Romanian practice uses series prefix + sequential number, current system only has single "number" field
• Seller registration number (J number / Nr. Reg. Com.) — required for Romanian B2B invoices
• Seller bank details (IBAN, bank name) — commonly required on Romanian invoices
• VAT breakdown per rate — show subtotals grouped by VAT rate (21%, 9%, 0%) when multiple rates exist
• Amount in words (Romanian) — "una mie doua sute lei" — common practice on Romanian invoices
• Customer full address — currently partial display
• Customer tax ID (CUI/CIF) — not shown on PDF
• Customer registration number — not shown on PDF
• EN16931 line-level fields — period_start/end, domain_name, seller_item_id should appear on PDF line items
• Payment terms — structured display beyond prose text
• Discount/allowance — line-level and document-level discounts not shown

Missing for Romanian Law

• Fiscal invoice disclaimer — partial, needs review for exact legal wording
• Reverse charge notation — for intra-EU B2B transactions, "taxare inversa" must appear
• Exchange rate — if invoice currency differs from RON, exchange rate must be shown per ANAF rules
• Delegate info — for physical transport/delivery invoices (may not apply to hosting)

Technical Notes

• Current implementation: services/platform/apps/billing/pdf_generators.py (ReportLab)
• PDF views: services/platform/apps/billing/views.py (invoice_pdf ~line 1220, proforma_pdf ~line 1126)
• Consider migrating to WeasyPrint or xhtml2pdf for HTML-based PDF templates (easier to maintain than coordinate-based ReportLab)
• For PDF/A-3 with embedded XML, consider using pikepdf or pdfrw to attach the UBL XML after generation
• EN16931 line-level fields (domain_name, seller_item_id, period) already exist on the models — just need to be rendered in PDF

Related

• EN16931 field additions implemented in the codebase (domain_name, seller_item_id, period_start/end on InvoiceLine/ProformaLine)
• XML builder (efactura/xml_builder.py) already generates compliant UBL XML — needs to be embedded in PDF