fix(reconcile): symbol을 슬롯 생성 시 함께 저장 + NULL 슬롯 backfill

reconcile_universe 시그니처를 set[str] → dict[str, str] (portfolio_id→symbol)로 변경.
기존 symbol=NULL 슬롯은 다음 reconcile 실행 시 자동 backfill.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/application/test_messaging_event_publisher.py

@@ -25,7 +25,7 @@ def _wire(transport):
     outbox = InMemoryLedgerOutbox()
     ledger = InMemoryLedger()
     writer = LedgerWriter(outbox, ledger)
-    repo.reconcile_universe("default", {"PF-1"})
+    repo.reconcile_universe("default", {"PF-1": "SYM"})
 
     def emitter(event_type: str, slot) -> None:
         rk = topology.event_routing_key(event_type, slot.strategy_id or "_", slot.symbol or "_")