feat(bat-020): daemon-scoped 브로커 인증정보 (REQ-046~051)

- REQ-046: BrokerCredential.daemon_id 필드 추가, repository 시그니처 갱신
- REQ-047: alembic revision Step1(daemon_id NULL 컬럼 추가), Step2(PK 전환, dev/test DB 한정)
- REQ-048: backfill-daemon-id CLI subcommand (--dry-run/--force/--broker 지원)
- REQ-049: GET /v1/user_info daemon-scoped 단일 broker 전달 (brokers→broker, daemon_id 없으면 403)
- REQ-050: credential set/list/remove --daemon-id 옵션 추가 (daemon_slug 해석)
- REQ-051: 보안 격리 - service-token 전용, web 토큰 차단, 로그 app_secret 비노출, graceful degrade

비가역 결재 게이트: REQ-047 Step2(PK 전환)는 사장님 결재 후 실DB 적용 예정(이 batch는 revision 코드만)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VHtM5Ws1HKCKQ4JGi6GRb5

app/schemas/v1.py

@@ -96,7 +96,7 @@ class UserInfoResponse(BaseModel):
     daemon_id: str | None = None
     trading_enabled: bool
     disabled_reason: str | None = None
-    brokers: list[BrokerItem] = []
+    broker: BrokerItem | None = None
     # Additional informational fields
     user_id: str
     username: str