feat: add PaymentInfo support for Platnosc element

[samupl]

Summary

Adds support for the <Platnosc> element (payment details) in the Fa section of the FA(3) schema.

What's new

• PaymentInfo model with:
  • is_paid: bool — emits <Zaplacono>1</Zaplacono> flag when True
  • payment_date — <DataZaplaty>
  • due_date + due_description — <TerminPlatnosci> / <Termin> / <TerminOpis>
  • method — <FormaPlatnosci> (use PAYMENT_METHOD_* constants)
  • bank_account_number — <RachunekBankowy> / <NrRB>
• Payment method constants matching TFormaPlatnosci enum:
  • PAYMENT_METHOD_CASH (1), CARD (2), VOUCHER (3), CHECK (4), CREDIT (5), BANK_TRANSFER (6), MOBILE (7)
• Optional payment_info field on InvoiceData
• XML serializer _build_payment_info() emitting elements in schema order: Zaplacono → DataZaplaty → TerminPlatnosci → FormaPlatnosci → RachunekBankowy

Note on Zaplacono

Initial attempt treated Zaplacono as a decimal amount, which KSeF rejected:

The value '100.00' is invalid according to its datatype 'TWybor1' - The string '100.00' is not a valid SByte value.

TWybor1 is a flag type accepting only value 1. The actual paid amount is implied from P_15 when the flag is set.

Test plan

• Existing 36 library tests pass
• Verified against KSeF test environment with 9 invoice variations (paid/unpaid, various methods)