feat(publish): P1-1 digest 발행 read API scaffold(503)

기존 /published 라우터에 GET /published/digest 추가 — _verify_token(Bearer)
+ FeedResponse 엔벨로프 재사용(신규 라우터 X). DIGEST_PUBLISH_ENABLED 플래그
(기본 false=inert): off=503 "not enabled", on+projection 미구현=503. 실데이터·시크릿 0.
검증: 무토큰 401·잘못된 토큰 403·유효+off 503·기존 /feed 200 무회귀.
docsrv-viewer-publish 트랙 (plan viewer-daily-report P1-1).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -212,6 +212,7 @@ services:
       - INTERNAL_WORKER_TOKEN=${INTERNAL_WORKER_TOKEN}
       # docsrv-viewer-publish: 발행 워커/저작 enqueue 게이트(기본 false=inert) + 뷰어↔DS feed Bearer.
       - STUDY_PUBLISH_ENABLED=${STUDY_PUBLISH_ENABLED:-false}
+      - DIGEST_PUBLISH_ENABLED=${DIGEST_PUBLISH_ENABLED:-false}
       - VIEWER_SYNC_TOKEN=${VIEWER_SYNC_TOKEN:-}
       # study-to-viewer P2: 뷰어 write-back ingest 게이트(기본 false=inert, 검증 후 점등).
       - STUDY_INGEST_ENABLED=${STUDY_INGEST_ENABLED:-false}