feat: REQ-028 add display_name to GET /v1/portfolios response

- Portfolio domain model + PortfolioItem schema에 display_name 필드 추가
- _SELECT_PORTFOLIO에 a.display_name JOIN 보강, _portfolio_to_item 매퍼 전달
- REQ-028 CHG-20260601-01/02: Implemented → Approved → Implemented
  - AC-13 신설 (display_name == joined assets.display_name)
- BAT-009 verification.md: AC-13 매핑 + augmentation history 누적
- REL-001 verification checklist 재실행 evidence 갱신 (368 PASSED, 로컬 smoke 5건 통과)
- tests/test_v1.py: dead 중복 GET /v1/assets 섹션 제거 (두 번째 동명 정의가 첫 번째를 override하던 잔재)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/test_v1_portfolios_list.py

@@ -26,8 +26,14 @@ def _make_settings(**kwargs) -> Settings:
     return Settings(allowed_origins_raw="http://localhost:3000", **kwargs)
 
 
-def _make_asset(markets: list[str] | None = None) -> Asset:
-    return Asset(asset_id=_ASSET_UUID, symbol="005930", markets=markets or ["KRX"], status="active")
+def _make_asset(markets: list[str] | None = None, display_name: str | None = "삼성전자") -> Asset:
+    return Asset(
+        asset_id=_ASSET_UUID,
+        symbol="005930",
+        markets=markets or ["KRX"],
+        display_name=display_name,
+        status="active",
+    )
 
 
 def _make_portfolio(
@@ -35,6 +41,7 @@ def _make_portfolio(
     owner: str = _USER_ID,
     status: str = "enabled",
     daemon_id: str | None = None,
+    display_name: str | None = "삼성전자",
 ) -> Portfolio:
     return Portfolio(
         portfolio_id=portfolio_id,
@@ -42,6 +49,7 @@ def _make_portfolio(
         status=status,
         name=f"p-{portfolio_id[:4]}",
         symbol="005930",
+        display_name=display_name,
         asset_id=_ASSET_UUID,
         owner_user_id=owner,
         cash_balance=0.0,