feat(fx): 환율 캐시 시각 노출 — updatedAt 응답 추가

- FxService: Cached 레코드에 fetchedAt(Instant) 추가, getCachedAt() 공개
- FxController: GET /api/fx/rate 응답에 updatedAt(ISO 문자열) 포함

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/main/java/com/sb/web/account/controller/FxController.java

@@ -7,6 +7,7 @@ import org.springframework.web.bind.annotation.RequestParam;
 import org.springframework.web.bind.annotation.RestController;
 
 import java.math.BigDecimal;
+import java.time.Instant;
 import java.util.HashMap;
 import java.util.Map;
 
@@ -28,10 +29,12 @@ public class FxController {
     public Map<String, Object> rate(@RequestParam String from,
                                     @RequestParam(defaultValue = "KRW") String to) {
         BigDecimal rate = fxService.getRate(from, to);
+        Instant updatedAt = fxService.getCachedAt(from);
         Map<String, Object> res = new HashMap<>();
         res.put("from", from == null ? null : from.toUpperCase());
         res.put("to", to == null ? null : to.toUpperCase());
         res.put("rate", rate); // 조회 실패 시 null
+        res.put("updatedAt", updatedAt != null ? updatedAt.toString() : null);
         return res;
     }
 }