feat: consolidate ledger trade writes into portfolio_ledger

- add ledger consolidation migration and remove ledger_trades usage
- return portfolio state after v1 ledger trade writes
- deprecate legacy buy/sell endpoints with deprecation headers
- add BAT-016, DCY-006, and REQ-036~038 documentation
- update repository models and tests for unified ledger entries

docs/discovery/index.md

@@ -11,3 +11,4 @@
 | dcy-003_20260520_user-portfolio-mutation-v1-api-migration | 20260520 | User Portfolio Mutation v1 API 이전 및 path 명명 분리 (/v1/portfolios* 신설) | [Discovery](./dcy-003_20260520_user-portfolio-mutation-v1-api-migration.md) | confirmed |
 | dcy-004_20260520_token-issuance-and-lifecycle-overhaul | 20260520 | 토큰 발급 및 관리 전면 정비 — client token 폐지, service-token rotate 정상화, 발급 권한 매트릭스, 만료·회전·자동 revoke 정책 | [Discovery](./dcy-004_20260520_token-issuance-and-lifecycle-overhaul.md) | confirmed |
 | dcy-005_20260601_rel-001-feedback-cleanup | 20260601 | REL-001 Feedback 정합화 — ledger_trades.asset_id 명명 / session no_expiry enforcement / health db_enabled | [Discovery](./dcy-005_20260601_rel-001-feedback-cleanup.md) | confirmed |
+| dcy-006_20260605_ledger-trade-write-portfolio-state-return | 20260605 | ledger trade write 회계 책임 단일화 — 응답에 portfolio 상태 포함 + portfolio_ledger/ledger_trades 테이블 통합 (issue #8) | [Discovery](./dcy-006_20260605_ledger-trade-write-portfolio-state-return.md) | confirmed |