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/MemberMapper.xml

@@ -0,0 +1,49 @@
+<?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.auth.mapper.MemberMapper">
+
+    <resultMap id="memberResultMap" type="com.sb.web.auth.domain.Member">
+        <id property="id" column="id"/>
+        <result property="loginId" column="login_id"/>
+        <result property="password" column="password"/>
+        <result property="name" column="name"/>
+        <result property="email" column="email"/>
+        <result property="provider" column="provider"/>
+        <result property="providerId" column="provider_id"/>
+        <result property="role" column="role"/>
+        <result property="status" column="status"/>
+        <result property="createdAt" column="created_at"/>
+        <result property="updatedAt" column="updated_at"/>
+    </resultMap>
+
+    <sql id="columns">
+        id, login_id, password, name, email, provider, provider_id, role, status, created_at, updated_at
+    </sql>
+
+    <select id="findById" parameterType="long" resultMap="memberResultMap">
+        SELECT <include refid="columns"/> FROM member WHERE id = #{id}
+    </select>
+
+    <select id="findByLoginId" parameterType="string" resultMap="memberResultMap">
+        SELECT <include refid="columns"/> FROM member WHERE login_id = #{loginId}
+    </select>
+
+    <select id="findByProvider" resultMap="memberResultMap">
+        SELECT <include refid="columns"/>
+        FROM member
+        WHERE provider = #{provider} AND provider_id = #{providerId}
+    </select>
+
+    <select id="countByLoginId" parameterType="string" resultType="int">
+        SELECT COUNT(*) FROM member WHERE login_id = #{loginId}
+    </select>
+
+    <insert id="insert" parameterType="com.sb.web.auth.domain.Member"
+            useGeneratedKeys="true" keyProperty="id">
+        INSERT INTO member (login_id, password, name, email, provider, provider_id, role, status, created_at, updated_at)
+        VALUES (#{loginId}, #{password}, #{name}, #{email},
+                #{provider}, #{providerId}, #{role}, #{status}, NOW(), NOW())
+    </insert>
+
+</mapper>