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, {})