Fix ETTN creation on live Checkbox

- prepayment as DISCOUNT: live API rejects PRE_PAYMENT with 400 third_party.generic
- normalize lowercase ETTN statuses returned by the live API
- show Checkbox error code instead of bare 'Internal Server Error'
- page size 50 for ETTN list, 429 rate limit is retryable, pause polling on it
- phone in 380XXXXXXXXX format like portal receipts

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/tests/test_receipts_service.py

@@ -117,7 +117,7 @@ class TestBuildBody:
             }
         ]
         assert "discounts" not in rb
-        assert rb["delivery"] == {"phone": "+380501112233", "emails": ["a@b.ua"]}
+        assert rb["delivery"] == {"phone": "380501112233", "emails": ["a@b.ua"]}
 
     def test_prepayment_line_and_order_discounts_and_tax(self) -> None:
         order = _order(
@@ -132,10 +132,8 @@ class TestBuildBody:
         assert rb["goods"][0]["discounts"] == [
             {"type": "DISCOUNT", "mode": "VALUE", "value": 10000}
         ]
-        assert [(d["type"], d["value"]) for d in rb["discounts"]] == [
-            ("DISCOUNT", 5000),
-            ("PRE_PAYMENT", 20000),
-        ]
+        # Скидка заказа 50 ₴ + предоплата 200 ₴ — одной «Знижкой», без PRE_PAYMENT.
+        assert [(d["type"], d["value"]) for d in rb["discounts"]] == [("DISCOUNT", 25000)]
         assert rb["payments"][0]["value"] == 85000
 
     def test_all_goods_with_prices_quantities_and_sums(self) -> None:
@@ -166,7 +164,7 @@ class TestBuildBody:
         goods_total = sum(svc._line_sum(g["good"]["price"], g["quantity"]) for g in rb["goods"])
         assert goods_total - 20000 == rb["payments"][0]["value"] == 130049
         assert rb["discounts"] == [
-            {"type": "PRE_PAYMENT", "mode": "VALUE", "value": 20000, "name": "Передоплата"}
+            {"type": "DISCOUNT", "mode": "VALUE", "value": 20000, "name": "Знижка"}
         ]
 
     def test_fractional_quantity(self) -> None: