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/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>