feat: 支付手续费功能

- 支持提供商级别和渠道级别手续费率配置（FEE_RATE_PROVIDER_* / FEE_RATE_*）
- 用户多付手续费，到账金额不变（充值 ¥100 + 1.6% = 实付 ¥101.60）
- 前端显示手续费明细和实付金额
- 退款时按实付金额退款，余额扣减到账金额

src/lib/payment/registry.ts

@@ -30,6 +30,12 @@ export class PaymentProviderRegistry {
     const provider = this.providers.get(type as PaymentType);
     return provider?.defaultLimits?.[type];
   }
+
+  /** 获取指定渠道对应的提供商 key（如 'easypay'、'stripe'） */
+  getProviderKey(type: string): string | undefined {
+    const provider = this.providers.get(type as PaymentType);
+    return provider?.providerKey;
+  }
 }
 
 export const paymentRegistry = new PaymentProviderRegistry();

src/lib/payment/types.ts

@@ -62,6 +62,7 @@ export interface MethodDefaultLimits {
 /** Common interface that all payment providers must implement */
 export interface PaymentProvider {
   readonly name: string;
+  readonly providerKey: string;
   readonly supportedTypes: PaymentType[];
   /** 各渠道默认限额，key 为 PaymentType（如 'alipay'），可被环境变量覆盖 */
   readonly defaultLimits?: Record<string, MethodDefaultLimits>;