feat(invoice): separate date of taxable supply

Needed for accounting reasons.

Author: nijel

weblate_web/invoices/models.py

@@ -285,6 +285,10 @@ class Invoice(models.Model):  # noqa: PLR0904
         blank=True,
         help_text="Due date / Quote validity, keep blank unless specific terms are needed",
     )
+    tax_date = models.DateField(
+        blank=True,
+        help_text="Date of taxable supply, keep blank for issue date",
+    )
 
     kind = models.IntegerField(choices=InvoiceKind)
     category = models.IntegerField(
@@ -355,6 +359,8 @@ def save(  # type: ignore[override]
         if self.extra is None:
             self.extra = {}
         extra_fields: list[str] = []
+        if not self.tax_date:
+            self.tax_date = self.issue_date
         if not self.due_date:
             self.due_date = self.issue_date + datetime.timedelta(
                 days=self.get_due_delta()

weblate_web/invoices/templates/invoice-template.html

@@ -247,7 +247,7 @@ <h2>Issued by</h2>
             <th>Contact</th>
           </tr>
           <tr>
-            <td>{{ invoice.issue_date|date }}</td>
+            <td>{{ invoice.tax_date|date }}</td>
             <td>21668027</td>
             <td>Michal Čihař</td>
             <td>C 52324/KSUL Krajský soud v Ústí nad Labem</td>