fix(kakao): Kakao SDK v2 authorize 방식으로 전환 + code 교환 엔드포인트

- Kakao SDK v2.7에서 Auth.login() 제거 → authorize() 리다이렉트 플로우 사용
- services/kakao.ts: kakaoWebLoginStart() redirect 시작
- RootNavigator: 웹 URL의 code + state=kakao_login 감지 → kakaoCodeLogin 호출
- useAuthStore.kakaoCodeLogin 추가 (authApi.kakaoCode 호출)
- 백엔드 /auth/kakao/code: REST API 키로 code→access_token 교환 후 기존 processKakaoLogin 재사용
- api-secrets에 kakaoRestApiKey/kakaoClientSecret 슬롯 추가
- 환경변수: KAKAO_REST_API_KEY 서버에 저장 완료

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

deploy/k8s/api.yaml

@@ -78,6 +78,10 @@ spec:
               valueFrom: { secretKeyRef: { name: api-secrets, key: codefClientId, optional: true } }
             - name: CODEF_CLIENT_SECRET
               valueFrom: { secretKeyRef: { name: api-secrets, key: codefClientSecret, optional: true } }
+            - name: KAKAO_REST_API_KEY
+              valueFrom: { secretKeyRef: { name: api-secrets, key: kakaoRestApiKey, optional: true } }
+            - name: KAKAO_CLIENT_SECRET
+              valueFrom: { secretKeyRef: { name: api-secrets, key: kakaoClientSecret, optional: true } }
           readinessProbe:
             httpGet:
               path: /health