fix(payments): stop doubling the pack size in buyer mail

Catalog titles already name the pack ("50 фишек"), so appending the chip count
produced "50 фишек — 50 фишек, 79.00 ₽". The letter now carries the title
exactly as the storefront shows it, then the price.

The tax receipt is unaffected: it states the quantity in the wording the tax
service wants, which is a separate surface with separate requirements.

backend/internal/inttest/mynalog_test.go

@@ -661,6 +661,11 @@ func TestMyNalogBuyerMails(t *testing.T) {
 	if !strings.Contains(first[0].Text, "не чек") {
 		t.Errorf("the purchase letter does not say it is not the receipt:\n%s", first[0].Text)
 	}
+	// The catalog title already names the pack size, so appending it again read as
+	// "50 фишек — 50 фишек". The line is the title verbatim, then the price, and nothing else.
+	if !strings.Contains(first[0].Text, "Покупка совершена: test pack, 149.00 ₽.") {
+		t.Errorf("the purchase line is not the catalog title plus the price:\n%s", first[0].Text)
+	}
 
 	// 2. The receipt letter, once the income is filed, carrying the printable link.
 	if _, err := exp.RunBatch(ctx, time.Minute); err != nil {