feat(bat-021): REQ-047 Step2 동반 코드 수정 + 실 DB 통합검증 + 런북

- ensure_schema() broker_credentials: daemon_id NOT NULL, PK=(daemon_id,broker)로 갱신
- _row_to_broker_credential: Step2 컬럼 순서(daemon_id 선두)로 갱신
- upsert_broker_credential: ON CONFLICT (daemon_id,broker)로 수정, fallback try/except 제거
- list_broker_credentials: fallback 제거, SELECT 컬럼 순서 갱신
- test_bat020_broker_credential_domain.py: AC-3/AC-4 행 순서를 Step2 포맷으로 갱신
- tests/test_bat021_step2_ensure_schema_and_upsert.py: 12개 신규 테스트
- docs/runbooks/bat-021-req047-step2-migration-runbook.md: 운영 적용 런북
- docs/batches/bat-021: batch 문서(index/planning/design/implementation/verification)

실 PostgreSQL 통합검증(AC-1~7 전건 PASS):
  Step1 upgrade → backfill → Step2 upgrade → downgrade 시나리오 포함
  NULL 사전검증(upgrade 거부) 및 downgrade 안전장치(중복 시 RuntimeError) 확인

521 passed, 0 failed (기존 509 + 신규 12)

docs/batches/index.md

@@ -27,4 +27,5 @@
 | BAT-017 | standard | released | REQ-039, REQ-040, REQ-041, REQ-042 | dcy-007 | [bat-017](./bat-017_20260618_trade-history-and-pnl-api/) |
 | BAT-018 | batch-lite | release-candidate | REQ-043 | dcy-008 | [bat-018](./bat-018_20260625_holdings-cost-basis-endpoint/) |
 | BAT-019 | standard | confirmed | REQ-044, REQ-045 | dcy-009 | [bat-019](./bat-019_20260627_assets-read-permission-and-daemon-list/) |
-| BAT-020 | standard | draft-batch-verification | REQ-046, REQ-047, REQ-048, REQ-049, REQ-050, REQ-051 | dcy-010 | [bat-020](./bat-020_20260628_daemon-scoped-broker-credential/) |
+| BAT-020 | standard | release-candidate | REQ-046, REQ-047, REQ-048, REQ-049, REQ-050, REQ-051 | dcy-010 | [bat-020](./bat-020_20260628_daemon-scoped-broker-credential/) |
+| BAT-021 | standard | in-progress | REQ-047 | dcy-010 | [bat-021](./bat-021_20260628_req047-step2-integration-and-runbook/) |