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/app/schemas/checkbox.py

@@ -7,7 +7,7 @@
 
 from __future__ import annotations
 
-from pydantic import BaseModel, ConfigDict, Field
+from pydantic import BaseModel, ConfigDict, Field, field_validator
 
 
 class EttnStatus:
@@ -30,3 +30,10 @@ class EttnOut(BaseModel):
     total_sum: int | None = Field(default=None, alias="totalSum")
     receipt_id: str | None = Field(default=None, alias="receiptId")
     raw_error: str | None = Field(default=None, alias="rawError")
+
+    # В OpenAPI статусы заглавные, а боевой API отдаёт строчные (`"done"`,
+    # `"created"`) — приводим к одному регистру, иначе статусы не сопоставятся.
+    @field_validator("status", mode="before")
+    @classmethod
+    def _upper_status(cls, value: object) -> object:
+        return value.upper() if isinstance(value, str) else value