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_update_thresholds_handler.py

@@ -45,7 +45,7 @@ def _holding(service, repo, *, symbol="005930", stop_loss_price=None,
              take_profit_price=None, forced_close_at=None):
     from uuid import uuid4
     pid = f"pf-{uuid4().hex[:8]}"
-    repo.reconcile_universe("default", {pid})
+    repo.reconcile_universe("default", {pid: "SYM"})
     slot = repo.find_slot_by_portfolio("default", pid)
     pending = service.transit_slot(
         slot.slot_id, SlotState.INACTIVE, SlotState.PENDING_BUY,
@@ -111,7 +111,7 @@ def test_explicit_null_clears(handler, service, repo):
 
 def test_non_holding_ignored(handler, service, repo, watchdog):
     """AC-6: non-holding slot → ignored + not_holding reason, no watchdog reset."""
-    repo.reconcile_universe("default", {"pf-inactive"})
+    repo.reconcile_universe("default", {"pf-inactive": "SYM"})
     inactive = repo.find_slot_by_portfolio("default", "pf-inactive")
     resp = handler(_req(slot_id=str(inactive.slot_id), stop_loss_price=50000))
     assert resp.payload["status"] == "ignored"