feat: 게시판 신고 — 누적 5건 시 블라인드(관리자만 열람)

- report 테이블(회원당 대상별 1회), comment.blocked 컬럼
- POST /board/posts|comments/{id}/report, 본인 글/댓글 신고 불가
- 글: 5건이면 blocked(기존 열람제한 재사용) / 댓글: blocked → 비관리자 본문 숨김
- 삭제·탈퇴 시 신고 정리

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

src/main/resources/mapper/ReportMapper.xml

@@ -0,0 +1,23 @@
+<?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.ReportMapper">
+
+    <insert id="insertIgnore">
+        INSERT IGNORE INTO report (target_type, target_id, member_id, reason, created_at)
+        VALUES (#{targetType}, #{targetId}, #{memberId}, #{reason}, NOW())
+    </insert>
+
+    <select id="countByTarget" resultType="int">
+        SELECT COUNT(*) FROM report WHERE target_type = #{targetType} AND target_id = #{targetId}
+    </select>
+
+    <delete id="deleteByTarget">
+        DELETE FROM report WHERE target_type = #{targetType} AND target_id = #{targetId}
+    </delete>
+
+    <delete id="deleteByMember">
+        DELETE FROM report WHERE member_id = #{memberId}
+    </delete>
+
+</mapper>