feat(backend): #357 DDG → Naver Search 정식 API + DDG 폴백

- WebSearchService 신규 (Naver webkr.json 우선, 키 미설정/실패 시 DDG)
- RestaurantController.searchTabling/searchCatchtable 내부 호출 교체
- 인라인 DDG 80줄 제거, 미사용 import 정리
- app.naver.client-id/secret 추가 (env: NAVER_CLIENT_ID/SECRET)
- k8s secrets template에 NAVER 키 항목

Refs: #357 (close)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend-java/src/main/resources/application.yml

@@ -56,6 +56,11 @@ app:
     youtube-api-key: ${YOUTUBE_DATA_API_KEY}
     client-id: ${GOOGLE_CLIENT_ID:635551099330-2l003d3ernjmkqavd4f6s78r8r405iml.apps.googleusercontent.com}
 
+  # #357 — Naver Search API (Tabling/Catchtable URL 매칭). 미설정 시 DDG 폴백.
+  naver:
+    client-id: ${NAVER_CLIENT_ID:}
+    client-secret: ${NAVER_CLIENT_SECRET:}
+
   cache:
     ttl-seconds: 600