feat: 멤버십(무료/유료) 게이팅 — 백엔드

- member.plan(FREE/PREMIUM) 컬럼 추가 + SessionUser/MemberResponse 노출
- auth_session 백업에 plan 저장(세션 복원 시 유료 상태 유지)
- 관리자 회원 plan 변경 API (PUT /api/admin/members/{id}/plan)
- PremiumInterceptor 로 유료 전용 경로 차단(통계/예산/고정지출/태그/OCR/투자/카드알림/백업)
  · 관리자는 플랜과 무관하게 허용, 무료 회원은 403 PREMIUM_REQUIRED
- WebConfigTest: 유료 경로 등록 회귀 가드 추가

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

src/main/resources/mapper/MemberMapper.xml

@@ -13,13 +13,14 @@
         <result property="providerId" column="provider_id"/>
         <result property="googleId" column="google_id"/>
         <result property="role" column="role"/>
+        <result property="plan" column="plan"/>
         <result property="status" column="status"/>
         <result property="createdAt" column="created_at"/>
         <result property="updatedAt" column="updated_at"/>
     </resultMap>
 
     <sql id="columns">
-        id, login_id, password, name, email, provider, provider_id, google_id, role, status, created_at, updated_at
+        id, login_id, password, name, email, provider, provider_id, google_id, role, plan, status, created_at, updated_at
     </sql>
 
     <select id="findById" parameterType="long" resultMap="memberResultMap">
@@ -82,6 +83,10 @@
         UPDATE member SET role = #{role}, updated_at = NOW() WHERE id = #{id}
     </update>
 
+    <update id="updatePlan">
+        UPDATE member SET plan = #{plan}, updated_at = NOW() WHERE id = #{id}
+    </update>
+
     <update id="updateStatus">
         UPDATE member SET status = #{status}, updated_at = NOW() WHERE id = #{id}
     </update>