feat: 支付渠道每日限额（渠道维度全平台统计）

- config.ts：新增 MAX_DAILY_AMOUNT_ALIPAY/WXPAY/STRIPE（默认 alipay/wxpay 各 1w，stripe 不限）
- lib/order/limits.ts：getMethodDailyLimit + queryMethodLimits 共用工具，支持动态渠道兜底
- order/service.ts：createOrder 校验渠道限额，超限抛 METHOD_DAILY_LIMIT_EXCEEDED
- api/limits/route.ts：公开接口 GET /api/limits，返回各渠道今日用量/剩余/是否可用
- api/user/route.ts：config 响应中加入 methodLimits，前端一次请求即可获取限额状态
- PaymentForm.tsx：额度已满的渠道置灰 + 标注「今日额度已满」，无法选择
- pay/page.tsx：AppConfig 加 methodLimits，传给 PaymentForm，新增错误码映射

src/app/api/user/route.ts

@@ -1,6 +1,7 @@
 import { NextRequest, NextResponse } from 'next/server';
 import { getUser } from '@/lib/sub2api/client';
 import { getEnv } from '@/lib/config';
+import { queryMethodLimits } from '@/lib/order/limits';
 
 export async function GET(request: NextRequest) {
   const userId = Number(request.nextUrl.searchParams.get('user_id'));
@@ -10,7 +11,10 @@ export async function GET(request: NextRequest) {
 
   try {
     const env = getEnv();
-    const user = await getUser(userId);
+    const [user, methodLimits] = await Promise.all([
+      getUser(userId),
+      queryMethodLimits(env.ENABLED_PAYMENT_TYPES),
+    ]);
 
     return NextResponse.json({
       user: {
@@ -22,6 +26,7 @@ export async function GET(request: NextRequest) {
         minAmount: env.MIN_RECHARGE_AMOUNT,
         maxAmount: env.MAX_RECHARGE_AMOUNT,
         maxDailyAmount: env.MAX_DAILY_RECHARGE_AMOUNT,
+        methodLimits,
       },
     });
   } catch (error) {