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);