feat: 插件化支付渠道限额 — provider 自声明单笔/每日默认限额

- PaymentProvider 接口新增 defaultLimits（单笔 singleMax + 每日 dailyMax）
- EasyPay 默认：支付宝/微信各 单笔 ¥1000、每日 ¥10000
- Stripe 默认：不限额（0 = unlimited）
- getMethodDailyLimit / getMethodSingleLimit 优先读 env var，再回退 provider 默认
- queryMethodLimits 返回 singleMax，PaymentForm 按渠道动态调整最大单笔金额
- MAX_DAILY_AMOUNT_* 改为可选 env var 覆盖（不再有硬编码默认值）

src/lib/payment/registry.ts

@@ -1,4 +1,4 @@
-import type { PaymentProvider, PaymentType } from './types';
+import type { PaymentProvider, PaymentType, MethodDefaultLimits } from './types';
 
 export class PaymentProviderRegistry {
   private providers = new Map<PaymentType, PaymentProvider>();
@@ -24,6 +24,12 @@ export class PaymentProviderRegistry {
   getSupportedTypes(): PaymentType[] {
     return Array.from(this.providers.keys());
   }
+
+  /** 获取指定渠道的提供商默认限额（未注册时返回 undefined） */
+  getDefaultLimit(type: string): MethodDefaultLimits | undefined {
+    const provider = this.providers.get(type as PaymentType);
+    return provider?.defaultLimits?.[type];
+  }
 }
 
 export const paymentRegistry = new PaymentProviderRegistry();