feat: resolve and persist actual order venue

tests/adapters/persistence/test_postgresql_ledger_outbox.py

@@ -20,7 +20,7 @@ def _trade(execution_id="paper:1:10") -> LedgerTrade:
     return LedgerTrade(
         asset_id="PF-1", order_id="ORD-1", execution_id=execution_id,
         side="buy", qty=10, price=1000.0, fee=1.5, tax=0.0,
-        executed_at=datetime(2026, 6, 4, tzinfo=timezone.utc),
+        executed_at=datetime(2026, 6, 4, tzinfo=timezone.utc), venue="NXT",
     )
 
 
@@ -32,6 +32,7 @@ def test_enqueue_persists_and_is_idempotent(pg_clean):
     pending = outbox.list_pending()
     assert len(pending) == 1
     assert pending[0].status is WorkItemStatus.PENDING
+    assert pending[0].trade.venue == "NXT"
 
 
 def test_writer_completes_persisted_item(pg_clean):