test: validate ledger backfill flow

Co-authored-by: genichin <4282227+genichin@users.noreply.github.com>

tests/application/test_ledger_outbox.py

@@ -1,7 +1,7 @@
 """LedgerOutbox + LedgerWriter direct unit tests (REQ-019)."""
 from __future__ import annotations
 
-from datetime import datetime, timedelta, timezone
+from datetime import UTC, datetime, timedelta
 
 from trex_core.adapters.ledger.in_memory import InMemoryLedger
 from trex_core.application.ledger_outbox import (
@@ -17,7 +17,7 @@ def _trade(execution_id="paper:PAPER-1:10") -> LedgerTrade:
     return LedgerTrade(
         asset_id="PF-1", order_id="PAPER-1", execution_id=execution_id,
         side="buy", qty=10, price=1000.0,
-        executed_at=datetime(2026, 6, 4, tzinfo=timezone.utc),
+        executed_at=datetime(2026, 6, 4, tzinfo=UTC),
     )
 
 
@@ -57,7 +57,7 @@ def test_writer_marks_conflict_blocked_and_does_not_retry():
 def test_writer_retries_until_success():
     # dcy-015: retryable rows now back off; share an injected clock between the
     # outbox (list_pending "now") and the writer (next_retry_at) to advance past it.
-    now = {"t": datetime(2026, 6, 4, tzinfo=timezone.utc)}
+    now = {"t": datetime(2026, 6, 4, tzinfo=UTC)}
     outbox = InMemoryLedgerOutbox(clock=lambda: now["t"])
     ledger = InMemoryLedger(fail_n_then_succeed=1)
     writer = LedgerWriter(outbox, ledger, clock=lambda: now["t"])