feat: 외화 결제 입력 — 통화+환율→원화 환산, 원본 보존 + 환율 API

- account_entry에 currency/original_amount/exchange_rate 컬럼 추가(멱등 ALTER)
  amount는 환산 원화(표준값) 유지 → 통계·예산 무변경
- AccountEntry/Request/Response·매퍼·서비스에 외화 필드 전달
- FxService(open.er-api.com 무료·무인증, 통화별 일자 캐시) + FxController
  GET /api/fx/rate?from=USD&to=KRW → { from, to, rate }
- WebConfig: /api/fx/** 인증 보호
- (.gitignore: 로컬 시드 스크립트 제외 규칙)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -234,6 +234,9 @@ public class AccountService {
                 .walletId(req.getWalletId())
                 .toWalletId(transfer ? req.getToWalletId() : null)
                 .installmentMonths(installmentOf(req))
+                .currency(req.getCurrency())
+                .originalAmount(req.getOriginalAmount())
+                .exchangeRate(req.getExchangeRate())
                 .build();
         mapper.insert(entry);
         applyTags(entry.getId(), req.getTagIds(), memberId);
@@ -253,6 +256,9 @@ public class AccountService {
         entry.setWalletId(req.getWalletId());
         entry.setToWalletId(transfer ? req.getToWalletId() : null);
         entry.setInstallmentMonths(installmentOf(req));
+        entry.setCurrency(req.getCurrency());
+        entry.setOriginalAmount(req.getOriginalAmount());
+        entry.setExchangeRate(req.getExchangeRate());
         mapper.update(entry);
 
         mapper.deleteEntryTagsByEntryId(id);

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

@@ -0,0 +1,64 @@
+package com.sb.web.account.service;
+
+import com.fasterxml.jackson.databind.JsonNode;
+import lombok.extern.slf4j.Slf4j;
+import org.springframework.stereotype.Service;
+import org.springframework.web.client.RestClient;
+
+import java.math.BigDecimal;
+import java.time.LocalDate;
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+
+/**
+ * 환율 조회. open.er-api.com(무료·무인증) 사용. 통화(base)별로 하루 1회만 호출하고 캐시.
+ * 실패해도 예외를 던지지 않고 null(또는 직전 캐시) 반환.
+ */
+@Slf4j
+@Service
+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 final Map<String, Cached> cache = new ConcurrentHashMap<>();
+
+    /** from 통화 1단위 → to 통화 환율. 조회 실패 시 null. */
+    public BigDecimal getRate(String from, String to) {
+        if (from == null || to == null) return null;
+        String base = from.trim().toUpperCase();
+        String target = to.trim().toUpperCase();
+        if (base.isEmpty() || target.isEmpty()) return null;
+        if (base.equals(target)) return BigDecimal.ONE;
+        JsonNode rates = ratesFor(base);
+        if (rates == null) return null;
+        JsonNode r = rates.get(target);
+        return (r != null && r.isNumber()) ? r.decimalValue() : null;
+    }
+
+    private JsonNode ratesFor(String base) {
+        Cached c = cache.get(base);
+        LocalDate today = LocalDate.now();
+        if (c != null && c.date().equals(today)) return c.rates();
+        try {
+            JsonNode root = restClient.get()
+                    .uri(URL, base)
+                    .header("User-Agent", "Mozilla/5.0")
+                    .retrieve()
+                    .body(JsonNode.class);
+            if (root == null || !"success".equals(root.path("result").asText())) {
+                return c != null ? c.rates() : null;
+            }
+            JsonNode rates = root.path("rates");
+            if (rates.isMissingNode() || !rates.isObject()) {
+                return c != null ? c.rates() : null;
+            }
+            cache.put(base, new Cached(today, rates));
+            return rates;
+        } catch (Exception e) {
+            log.warn("환율 조회 실패 base={}: {}", base, e.toString());
+            return c != null ? c.rates() : null; // 실패 시 직전 캐시라도
+        }
+    }
+}