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/CommentMapper.xml

@@ -0,0 +1,42 @@
+<?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.board.mapper.CommentMapper">
+
+    <resultMap id="commentResultMap" type="com.sb.web.board.domain.Comment">
+        <id property="id" column="id"/>
+        <result property="postId" column="post_id"/>
+        <result property="authorId" column="author_id"/>
+        <result property="authorName" column="author_name"/>
+        <result property="content" column="content"/>
+        <result property="createdAt" column="created_at"/>
+    </resultMap>
+
+    <select id="findByPostId" parameterType="long" resultMap="commentResultMap">
+        SELECT id, post_id, author_id, author_name, content, created_at
+        FROM comment
+        WHERE post_id = #{postId}
+        ORDER BY id ASC
+    </select>
+
+    <select id="findById" parameterType="long" resultMap="commentResultMap">
+        SELECT id, post_id, author_id, author_name, content, created_at
+        FROM comment
+        WHERE id = #{id}
+    </select>
+
+    <insert id="insert" parameterType="com.sb.web.board.domain.Comment"
+            useGeneratedKeys="true" keyProperty="id">
+        INSERT INTO comment (post_id, author_id, author_name, content, created_at)
+        VALUES (#{postId}, #{authorId}, #{authorName}, #{content}, NOW())
+    </insert>
+
+    <delete id="delete" parameterType="long">
+        DELETE FROM comment WHERE id = #{id}
+    </delete>
+
+    <delete id="deleteByPostId" parameterType="long">
+        DELETE FROM comment WHERE post_id = #{postId}
+    </delete>
+
+</mapper>