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/AuthSessionMapper.xml

@@ -5,16 +5,16 @@
 
     <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)
+            (token, member_id, login_id, name, role, plan, provider, remember_me, expires_at, created_at)
         VALUES
-            (#{token}, #{memberId}, #{loginId}, #{name}, #{role}, #{provider},
+            (#{token}, #{memberId}, #{loginId}, #{name}, #{role}, #{plan}, #{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
+        SELECT token, member_id, login_id, name, role, plan, provider, remember_me, expires_at, created_at
         FROM auth_session
         WHERE token = #{token}
     </select>