feat: registration gravity from DB config instead of hardcoded

- Add registrationGravity to pricing config (default 50, adjustable via admin)
- Inject PricingService into UserService, read config for all 4 registration paths
- Expose registrationGravity in /member/plans response for frontend
- Login page fetches dynamic value from API, replaces hardcoded '50'
- Update user.vue gravity card copy

zhiyin-app/src/pages/user/user.vue

@@ -49,7 +49,7 @@
             </view>
             <text class="gravity-num">{{ memberInfo.gravity ?? 0 }}</text>
           </view>
-          <text class="gravity-hint">每次面试消耗 5 引力值 · 分享可获得更多</text>
+          <text class="gravity-hint">每次面试消耗 5 引力值 · 分享好友、贡献面经可获得更多</text>
         <view class="gravity-actions">
           <text class="gravity-btn share" @click="goSharePage">分享得引力值</text>
           <text class="gravity-btn contribute" @click="goContributePage">贡献面经</text>
@@ -150,7 +150,7 @@
           <view class="gp-method-icon-wrap"><text class="gp-method-icon">🛒</text></view>
           <view class="gp-method-info">
             <text class="gp-method-name">购买引力值</text>
-            <text class="gp-method-desc">小程序内直接购买，微信支付安全便捷</text>
+            <text class="gp-method-desc">¥5 一份，微信支付安全便捷</text>
           </view>
           <text class="gp-method-arrow">›</text>
         </view>