fix(c2c-bot): 修复 sharp 导入方式，解决水单生成 is not a function 错误

NestJS 使用 CommonJS 模块系统，`import sharp from 'sharp'` 编译后
生成 `sharp_1.default` 调用，但 sharp 没有 default export。
改为 `import * as sharp from 'sharp'` 修复。

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/services/trading-service/src/application/services/payment-proof.service.ts

@@ -1,7 +1,7 @@
 import { Injectable, Logger } from '@nestjs/common';
 import * as fs from 'fs';
 import * as path from 'path';
-import sharp from 'sharp';
+import * as sharp from 'sharp';
 
 export interface ProofData {
   orderNo: string;