feat: 가계부·게시판 백엔드 API 구현

- com.sb.web 패키지로 재구성: account / auth / board / user / admin / common
- 가계부(account): 내역(필터), 계좌·순자산, 예산, 분류, 정기 거래, 투자 포트폴리오
  (종목·매매 이력, 이동평균 평단·실현/평가손익)
- MyBatis + MariaDB + Redis 세션, BCrypt
- 스키마 자동 초기화(db/*.sql, 멱등), 사용자별 데이터 격리(member_id)
- 문서: docs/BACKEND.md, .env.example

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

src/main/resources/mapper/CategoryMapper.xml

@@ -0,0 +1,54 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
+        "https://mybatis.org/dtd/mybatis-3-mapper.dtd">
+<mapper namespace="com.sb.web.account.mapper.CategoryMapper">
+
+    <resultMap id="categoryResultMap" type="com.sb.web.account.domain.AccountCategory">
+        <id property="id" column="id"/>
+        <result property="memberId" column="member_id"/>
+        <result property="type" column="type"/>
+        <result property="name" column="name"/>
+        <result property="sortOrder" column="sort_order"/>
+        <result property="createdAt" column="created_at"/>
+    </resultMap>
+
+    <select id="findByMember" parameterType="long" resultMap="categoryResultMap">
+        SELECT id, member_id, type, name, sort_order, created_at
+        FROM account_category
+        WHERE member_id = #{memberId}
+        ORDER BY type, sort_order, name
+    </select>
+
+    <select id="findByIdAndMember" resultMap="categoryResultMap">
+        SELECT id, member_id, type, name, sort_order, created_at
+        FROM account_category
+        WHERE id = #{id} AND member_id = #{memberId}
+    </select>
+
+    <select id="countByName" resultType="int">
+        SELECT COUNT(*) FROM account_category
+        WHERE member_id = #{memberId} AND type = #{type} AND name = #{name}
+        <if test="excludeId != null">AND id != #{excludeId}</if>
+    </select>
+
+    <insert id="insert" parameterType="com.sb.web.account.domain.AccountCategory"
+            useGeneratedKeys="true" keyProperty="id">
+        INSERT INTO account_category (member_id, type, name, sort_order, created_at)
+        VALUES (#{memberId}, #{type}, #{name}, #{sortOrder}, NOW())
+    </insert>
+
+    <insert id="insertIgnore" parameterType="com.sb.web.account.domain.AccountCategory">
+        INSERT IGNORE INTO account_category (member_id, type, name, sort_order, created_at)
+        VALUES (#{memberId}, #{type}, #{name}, 0, NOW())
+    </insert>
+
+    <update id="update" parameterType="com.sb.web.account.domain.AccountCategory">
+        UPDATE account_category SET name = #{name}
+        WHERE id = #{id} AND member_id = #{memberId}
+    </update>
+
+    <delete id="delete">
+        DELETE FROM account_category WHERE id = #{id} AND member_id = #{memberId}
+    </delete>
+
+</mapper>