feat(api): P3b — locale 선택 + per-field fallback + served_locale echo

[thxforall]

Epic #683 · ADR-0007 D7 · plan docs/superpowers/plans/2026-06-18-locale-i18n-harness.md Phase 3.

api-server가 locale을 받아 콘텐츠 선택 + 필드단위 fallback(요청→ko→non-null) + served_locale 응답.

• Accept-Language + path locale 추출, DTO/handler 손봄
  의존: P3a(JSONB) 권장 선행.

[thxforall]

Closed automatically because PR #697 was merged into dev: #697