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_checkbox_client.py

@@ -148,3 +148,35 @@ async def test_bad_pin_raises() -> None:
     )
     with pytest.raises(CheckboxError, match="Невірний пін-код"):
         await _client().sign_in(CREDS)
+
+
+@respx.mock
+async def test_lowercase_status_from_live_api_is_normalized() -> None:
+    _signin()
+    respx.get(f"{BASE}/api/v1/ettn/e-1").mock(
+        return_value=Response(200, json={**ETTN, "status": "done"})
+    )
+    ettn = await _client().get_ettn(CREDS, "e-1")
+    assert ettn.status == "DONE"
+
+
+@respx.mock
+async def test_third_party_error_code_is_shown() -> None:
+    _signin()
+    respx.post(f"{BASE}/api/v1/ettn").mock(
+        return_value=Response(
+            400, json={"code": "third_party.generic", "message": "Internal Server Error"}
+        )
+    )
+    with pytest.raises(CheckboxError, match=r"third_party\.generic"):
+        await _client().create_ettn(CREDS, {})
+
+
+@respx.mock
+async def test_rate_limit_is_retryable() -> None:
+    _signin()
+    respx.post(f"{BASE}/api/v1/ettn").mock(
+        return_value=Response(429, json={"message": "Занадто часто виконуються запити"})
+    )
+    with pytest.raises(CheckboxUnavailableError):
+        await _client().create_ettn(CREDS, {})

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: