feat: add support for other tax special categories

Author: samupl

src/ksef/models/invoice_rows.py

@@ -1,15 +1,48 @@
 """Models for individual invoice rows/positions."""
 
-from typing import Sequence
+from typing import Literal, Sequence
 
 from pydantic import BaseModel
 
+# KSeF tax rate constants for the P_12 field (TStawkaPodatku in FA(3) schema)
+TAX_23 = 23
+TAX_22 = 22
+TAX_8 = 8
+TAX_7 = 7
+TAX_5 = 5
+TAX_4 = 4
+TAX_3 = 3
+TAX_0_KR = "0 KR"  # 0% domestic
+TAX_0_WDT = "0 WDT"  # 0% intra-Community supply
+TAX_0_EX = "0 EX"  # 0% export of goods
+TAX_ZW = "zw"  # exempt from tax
+TAX_OO = "oo"  # reverse charge
+TAX_NP_I = "np I"  # not subject to taxation — supply outside country territory
+TAX_NP_II = "np II"  # not subject to taxation — services per art. 100(1)(4)
+
+TaxRate = Literal[
+    23,
+    22,
+    8,
+    7,
+    5,
+    4,
+    3,
+    "0 KR",
+    "0 WDT",
+    "0 EX",
+    "zw",
+    "oo",
+    "np I",
+    "np II",
+]
+
 
 class InvoiceRow(BaseModel):
     """Single individual invoice position."""
 
     name: str  # P_7, nazwa (rodzaj) towaru lub usługi
-    tax: int  # P_12, stawka podatku
+    tax: TaxRate  # P_12, tax rate
 
 
 class InvoiceRows(BaseModel):