feat: 세션 DB 이중 저장 — Redis 유실/재시작에도 로그인 유지

- auth_session 테이블에 세션 영속 백업(login 시 Redis+DB 동시 저장)
- getSession: Redis 미스/장애 시 DB에서 복원 후 Redis 재수화(슬라이딩)
- logout: Redis+DB 동시 삭제, 만료분 일별 정리(@Scheduled)
- Redis 재시작으로 세션 전멸하던 주기적 로그아웃 해결

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

src/main/resources/mapper/AuthSessionMapper.xml

@@ -0,0 +1,34 @@
+<?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.AuthSessionMapper">
+
+    <insert id="insert" parameterType="com.sb.web.auth.domain.AuthSession">
+        INSERT INTO auth_session
+            (token, member_id, login_id, name, role, provider, remember_me, expires_at, created_at)
+        VALUES
+            (#{token}, #{memberId}, #{loginId}, #{name}, #{role}, #{provider},
+             #{rememberMe}, #{expiresAt}, NOW())
+        ON DUPLICATE KEY UPDATE
+            expires_at = #{expiresAt}
+    </insert>
+
+    <select id="findByToken" resultType="com.sb.web.auth.domain.AuthSession">
+        SELECT token, member_id, login_id, name, role, provider, remember_me, expires_at, created_at
+        FROM auth_session
+        WHERE token = #{token}
+    </select>
+
+    <update id="updateExpiry">
+        UPDATE auth_session SET expires_at = #{expiresAt} WHERE token = #{token}
+    </update>
+
+    <delete id="delete">
+        DELETE FROM auth_session WHERE token = #{token}
+    </delete>
+
+    <delete id="deleteExpired">
+        DELETE FROM auth_session WHERE expires_at &lt; #{now}
+    </delete>
+
+</mapper>