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;
     }
 }

src/main/java/com/sb/web/account/service/FxService.java

@@ -6,6 +6,7 @@ import org.springframework.stereotype.Service;
 import org.springframework.web.client.RestClient;
 
 import java.math.BigDecimal;
+import java.time.Instant;
 import java.time.LocalDate;
 import java.util.Map;
 import java.util.concurrent.ConcurrentHashMap;
@@ -21,9 +22,17 @@ public class FxService {
     private static final String URL = "https://open.er-api.com/v6/latest/{base}";
     private final RestClient restClient = RestClient.builder().build();
 
-    private record Cached(LocalDate date, JsonNode rates) {}
+    private record Cached(LocalDate date, Instant fetchedAt, JsonNode rates) {}
     private final Map<String, Cached> cache = new ConcurrentHashMap<>();
 
+    /** from 통화 1단위 → to 통화 환율. 조회 실패 시 null. */
+    /** 해당 통화 환율이 마지막으로 캐시된 시각. 캐시 없으면 null. */
+    public Instant getCachedAt(String from) {
+        if (from == null) return null;
+        Cached c = cache.get(from.trim().toUpperCase());
+        return c != null ? c.fetchedAt() : null;
+    }
+
     /** from 통화 1단위 → to 통화 환율. 조회 실패 시 null. */
     public BigDecimal getRate(String from, String to) {
         if (from == null || to == null) return null;
@@ -54,7 +63,7 @@ public class FxService {
             if (rates.isMissingNode() || !rates.isObject()) {
                 return c != null ? c.rates() : null;
             }
-            cache.put(base, new Cached(today, rates));
+            cache.put(base, new Cached(today, Instant.now(), rates));
             return rates;
         } catch (Exception e) {
             log.warn("환율 조회 실패 base={}: {}", base, e.toString());