feat(frontend): single pay-box positions + Zibll cover episodes/slides

Wire pay_box_position to box_top/top/bottom/box_bottom, render download or
paywall modules at those slots, and resolve featured_slide/attachment IDs
plus video episode chrome for single covers.

assets/admin/src/schema/sections/payment/order.js

@@ -57,7 +57,7 @@ export const orderSchema = {
       icon: 'fas fa-eye',
       fields: [
         { key: 'pay_mark', label: '付费标记', type: 'input', default: '付费' },
-        { key: 'pay_box_position', label: '付费框位置', type: 'radio', default: 'top', options: [{ label: '内容顶部（正文前）', value: 'top' }, { label: '内容底部（正文后）', value: 'bottom' }] },
+        { key: 'pay_box_position', label: '付费框位置', type: 'radio', default: 'top', options: [{ label: '文章框顶部（封面后）', value: 'box_top' }, { label: '正文顶部', value: 'top' }, { label: '正文底部', value: 'bottom' }, { label: '文章框底部（相关前）', value: 'box_bottom' }] },
         { key: 'pay_free_logged_show', label: '登录用户显示免费', type: 'switch', default: false },
         { key: 'payment_show_ip', label: '显示用户IP', type: 'switch', default: false },
         { key: 'payment_show_device', label: '显示设备信息', type: 'switch', default: false },

assets/admin/src/schema/sections/payment/paid.js

@@ -51,7 +51,7 @@ export const paidSchema = {
         { key: 'payment_currency_symbol', label: '货币符号', type: 'input', default: '￥', help: '商品价格前缀，如 ￥、$、¥' },
         { key: 'pay_show_user', label: '显示购买用户', type: 'switch', default: true },
         { key: 'pay_show_paycount', label: '显示购买次数', type: 'switch', default: true },
-        { key: 'pay_box_position', label: '付费框位置', type: 'radio', default: 'top', options: [{ label: '内容顶部（正文前）', value: 'top' }, { label: '内容底部（正文后）', value: 'bottom' }] },
+        { key: 'pay_box_position', label: '付费框位置', type: 'radio', default: 'top', options: [{ label: '文章框顶部（封面后）', value: 'box_top' }, { label: '正文顶部', value: 'top' }, { label: '正文底部', value: 'bottom' }, { label: '文章框底部（相关前）', value: 'box_bottom' }] },
       ],
     },
     {