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>

app/domain/models.py

@@ -107,6 +107,7 @@ class Portfolio:
     name: str
     symbol: str
     asset_id: str | None = None
+    display_name: str | None = None
     tags: list[str] | None = None
     tag_ids: list[str] | None = None
     owner_user_id: str | None = None