feat: 인앱 결제(Google Play) 스캐폴드 + 멤버십 만료 처리

- member.plan_expires_at + iap_purchase 테이블(중복 결제 방지)
- POST /api/billing/google/verify: 구매 검증 후 PREMIUM 부여/갱신(만료 연장), 세션 plan 즉시 동기화
- GET /api/billing/products: 구독 상품(월간/연간)
- 테스트 모드(billing.test-mode=true): test- 토큰 검증 없이 승인 → 전 과정 테스트 가능
- GooglePlayVerifier 인터페이스 + 스텁(실연동 지점), 실연동 시 구현체 교체
- 관리자 수동 plan 변경은 만료일 없이(영구), 결제는 만료일 부여
- 만료 자동 강등 스케줄러(매시), MemberResponse.planExpiresAt 노출

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

src/main/resources/db/member.sql

@@ -38,6 +38,25 @@ ALTER TABLE member ADD COLUMN IF NOT EXISTS profile_image MEDIUMTEXT NULL COMMEN
 -- 포인트(게시판 글/댓글 작성 보상). 기존 회원은 0 으로 시작 (멱등).
 ALTER TABLE member ADD COLUMN IF NOT EXISTS points BIGINT NOT NULL DEFAULT 0 COMMENT '활동 포인트' AFTER plan;
 
+-- 멤버십 만료일 (유료 구독). NULL = 만료 없음(관리자 영구 부여) 또는 무료. 만료 지나면 스케줄러가 FREE 로 강등.
+ALTER TABLE member ADD COLUMN IF NOT EXISTS plan_expires_at DATETIME NULL COMMENT '유료 멤버십 만료일시' AFTER plan;
+
+-- 인앱 결제 기록 (영수증 검증·중복 방지·감사)
+CREATE TABLE IF NOT EXISTS iap_purchase (
+    id             BIGINT       NOT NULL AUTO_INCREMENT,
+    member_id      BIGINT       NOT NULL,
+    platform       VARCHAR(20)  NOT NULL COMMENT 'GOOGLE_PLAY / TEST',
+    product_id     VARCHAR(100) NOT NULL COMMENT '상품 ID(premium_monthly 등)',
+    purchase_token VARCHAR(512) NOT NULL COMMENT 'Play 구매 토큰(또는 테스트 토큰)',
+    order_id       VARCHAR(120) NULL,
+    status         VARCHAR(20)  NOT NULL COMMENT 'VERIFIED / FAILED',
+    expires_at     DATETIME     NULL COMMENT '이 결제로 부여된 만료일',
+    created_at     DATETIME     NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    PRIMARY KEY (id),
+    UNIQUE KEY uk_iap_token (purchase_token),
+    KEY idx_iap_member (member_id)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
+
 -- 포인트 적립/차감 이력 (감사 + 일일 지급 한도 계산용).
 CREATE TABLE IF NOT EXISTS point_history (
     id         BIGINT      NOT NULL AUTO_INCREMENT,

src/main/resources/mapper/AuthSessionMapper.xml

@@ -27,6 +27,10 @@
         UPDATE auth_session SET name = #{name} WHERE token = #{token}
     </update>
 
+    <update id="updatePlan">
+        UPDATE auth_session SET plan = #{plan} WHERE token = #{token}
+    </update>
+
     <delete id="delete">
         DELETE FROM auth_session WHERE token = #{token}
     </delete>

src/main/resources/mapper/IapPurchaseMapper.xml

@@ -0,0 +1,29 @@
+<?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.billing.mapper.IapPurchaseMapper">
+
+    <resultMap id="iapResultMap" type="com.sb.web.billing.domain.IapPurchase">
+        <id property="id" column="id"/>
+        <result property="memberId" column="member_id"/>
+        <result property="platform" column="platform"/>
+        <result property="productId" column="product_id"/>
+        <result property="purchaseToken" column="purchase_token"/>
+        <result property="orderId" column="order_id"/>
+        <result property="status" column="status"/>
+        <result property="expiresAt" column="expires_at"/>
+        <result property="createdAt" column="created_at"/>
+    </resultMap>
+
+    <insert id="insert" parameterType="com.sb.web.billing.domain.IapPurchase"
+            useGeneratedKeys="true" keyProperty="id">
+        INSERT INTO iap_purchase (member_id, platform, product_id, purchase_token, order_id, status, expires_at, created_at)
+        VALUES (#{memberId}, #{platform}, #{productId}, #{purchaseToken}, #{orderId}, #{status}, #{expiresAt}, NOW())
+    </insert>
+
+    <select id="findByToken" resultMap="iapResultMap">
+        SELECT id, member_id, platform, product_id, purchase_token, order_id, status, expires_at, created_at
+        FROM iap_purchase WHERE purchase_token = #{purchaseToken}
+    </select>
+
+</mapper>

src/main/resources/mapper/MemberMapper.xml

@@ -16,6 +16,7 @@
         <result property="profileImage" column="profile_image"/>
         <result property="role" column="role"/>
         <result property="plan" column="plan"/>
+        <result property="planExpiresAt" column="plan_expires_at"/>
         <result property="points" column="points"/>
         <result property="status" column="status"/>
         <result property="createdAt" column="created_at"/>
@@ -23,7 +24,7 @@
     </resultMap>
 
     <sql id="columns">
-        id, login_id, password, name, email, provider, provider_id, google_id, google_picture, profile_image, role, plan, points, status, created_at, updated_at
+        id, login_id, password, name, email, provider, provider_id, google_id, google_picture, profile_image, role, plan, plan_expires_at, points, status, created_at, updated_at
     </sql>
 
     <select id="findById" parameterType="long" resultMap="memberResultMap">
@@ -96,8 +97,20 @@
         UPDATE member SET role = #{role}, updated_at = NOW() WHERE id = #{id}
     </update>
 
+    <!-- 관리자 수동 변경: 만료일 없이(영구) 설정/해제 -->
     <update id="updatePlan">
-        UPDATE member SET plan = #{plan}, updated_at = NOW() WHERE id = #{id}
+        UPDATE member SET plan = #{plan}, plan_expires_at = NULL, updated_at = NOW() WHERE id = #{id}
+    </update>
+
+    <!-- 결제로 멤버십 부여/갱신: plan + 만료일 -->
+    <update id="updateMembership">
+        UPDATE member SET plan = #{plan}, plan_expires_at = #{expiresAt}, updated_at = NOW() WHERE id = #{id}
+    </update>
+
+    <!-- 만료된 유료회원 자동 강등 (스케줄러) -->
+    <update id="downgradeExpired">
+        UPDATE member SET plan = 'FREE', updated_at = NOW()
+        WHERE plan = 'PREMIUM' AND plan_expires_at IS NOT NULL AND plan_expires_at &lt; NOW()
     </update>
 
     <update id="updateStatus">