feat: 套餐有效期支持日/周/月单位，订阅履约改用兑换码流程，UI层次感优化

- Prisma: SubscriptionPlan 新增 validityUnit 字段 (day/week/month)
- 新增 subscription-utils.ts 计算实际天数及格式化显示
- Sub2API client createAndRedeem 支持 subscription 类型 (group_id, validity_days)
- 订阅履约从 assignSubscription 改为 createAndRedeem，在 Sub2API 留痕
- 订单创建动态计算天数（月单位按自然月差值）
- 管理后台表单支持有效期数值+单位下拉
- 前端 ChannelCard 渠道卡片视觉层次优化（模型标签渐变、倍率突出、闪电图标）
- 按量付费 banner 改为渐变背景+底部倍率说明标签
- 帮助/客服信息区块添加到充值、订阅、支付全流程页面
- 移除系统配置独立页面入口，subscriptions API 返回用户信息

src/app/api/admin/subscription-plans/[id]/route.ts

@@ -34,6 +34,9 @@ export async function PUT(request: NextRequest, { params }: { params: Promise<{
     if (body.price !== undefined) data.price = body.price;
     if (body.original_price !== undefined) data.originalPrice = body.original_price;
     if (body.validity_days !== undefined) data.validityDays = body.validity_days;
+    if (body.validity_unit !== undefined && ['day', 'week', 'month'].includes(body.validity_unit)) {
+      data.validityUnit = body.validity_unit;
+    }
     if (body.features !== undefined) data.features = body.features;
     if (body.for_sale !== undefined) data.forSale = body.for_sale;
     if (body.sort_order !== undefined) data.sortOrder = body.sort_order;