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/resources/mapper/CategoryMapper.xml

@@ -51,4 +51,14 @@
         DELETE FROM account_category WHERE id = #{id} AND member_id = #{memberId}
     </delete>
 
+    <update id="updateSortOrder">
+        UPDATE account_category SET sort_order = #{sortOrder}
+        WHERE id = #{id} AND member_id = #{memberId}
+    </update>
+
+    <select id="maxSortOrder" resultType="java.lang.Integer">
+        SELECT MAX(sort_order) FROM account_category
+        WHERE member_id = #{memberId} AND type = #{type}
+    </select>
+
 </mapper>