feat: 분류 순서변경(reorder) + 월 예상 수입 설정 API

- 분류: sort_order reorder 엔드포인트, 생성 시 맨 뒤 배치
- 예산: account_setting 테이블 + 월 예상 수입 GET/PUT

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

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

@@ -0,0 +1,25 @@
+package com.sb.web.account.service;
+
+import com.sb.web.account.mapper.AccountSettingMapper;
+import lombok.RequiredArgsConstructor;
+import org.springframework.stereotype.Service;
+import org.springframework.transaction.annotation.Transactional;
+
+/**
+ * 사용자별 가계부 설정 서비스 (월 예상 수입 등).
+ */
+@Service
+@RequiredArgsConstructor
+public class AccountSettingService {
+
+    private final AccountSettingMapper settingMapper;
+
+    public Long getExpectedIncome(Long memberId) {
+        return settingMapper.findExpectedIncome(memberId);
+    }
+
+    @Transactional
+    public void setExpectedIncome(Long memberId, Long expectedIncome) {
+        settingMapper.upsertExpectedIncome(memberId, expectedIncome);
+    }
+}