fix: extract tax number, price basis quantity, spaced VAT IDs and contact email

- supplier/buyer: extract FC tax registration (BT-32/BT-49) into new
  tax_number field; previously only VA (USt-IdNr.) was read
- pflichtfelder: critical error only when neither vat_id nor tax_number
  is present (EN 16931 BR-CO-26)
- ustid: tolerate formatting whitespace inside VAT IDs (e.g. 'DE 140 978 617')
- line items: normalize unit price by basis quantity (BT-149), so a
  price per 100 units no longer inflates line totals by factor 100
- supplier email: also look in DefinedTradeContact (BT-42)
- rewrite xrechnung fallback test to assert only our own behavior,
  independent of factur-x version specifics

README.md

@@ -109,6 +109,7 @@ Extract ZUGFeRD data from a base64-encoded PDF file.
       "city": "Berlin",
       "country": "DE",
       "vat_id": "DE123456789",
+      "tax_number": null,
       "email": "billing@acme-corp.de"
     },
     "buyer": {
@@ -276,7 +277,7 @@ The service supports four validation checks:
 
 Validates that all critical invoice fields are present and non-empty:
 
-- **Critical errors:** invoice_number, invoice_date, supplier.name, supplier.vat_id, buyer.name, totals.net, totals.gross, totals.vat_total, line_items array, line item fields
+- **Critical errors:** invoice_number, invoice_date, supplier.name, supplier.vat_id (or supplier.tax_number), buyer.name, totals.net, totals.gross, totals.vat_total, line_items array, line item fields
 - **Warnings:** due_date, payment_terms.iban
 
 ### 2. betraege (Amount Calculations)
@@ -470,7 +471,7 @@ PORT=8000 python -m src.main
 Check that all required fields are present:
 - invoice_number
 - invoice_date (YYYY-MM-DD format)
-- supplier.name and supplier.vat_id
+- supplier.name and supplier.vat_id (or supplier.tax_number — EN 16931 BR-CO-26 requires one of both)
 - buyer.name
 - Non-zero totals (net, gross, vat_total)
 - At least one line item with description, quantity, and unit_price