fix(c2c): 修复分页参数类型转换问题导致的500错误

- 为 QueryC2cOrdersDto 和 QueryMyC2cOrdersDto 的 page/pageSize 字段添加 @Type(() => Number) 装饰器
- Query参数从URL获取时默认为字符串，需要显式转换为数字类型
- 添加 @IsInt() 验证确保参数为整数
- 修复 Prisma findMany take 参数期望 Int 但收到 String 的错误

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

backend/services/trading-service/src/api/dto/c2c.dto.ts

@@ -1,5 +1,6 @@
-import { IsString, IsIn, IsOptional, IsNumberString } from 'class-validator';
+import { IsString, IsIn, IsOptional, IsNumberString, IsInt } from 'class-validator';
 import { ApiProperty, ApiPropertyOptional } from '@nestjs/swagger';
+import { Type } from 'class-transformer';
 
 // ==================== C2C 订单类型和状态 ====================
 
@@ -110,12 +111,16 @@ export class QueryC2cOrdersDto {
   @IsIn(['BUY', 'SELL'])
   type?: 'BUY' | 'SELL';
 
-  @ApiPropertyOptional({ description: '页码' })
+  @ApiPropertyOptional({ description: '页码', default: 1 })
   @IsOptional()
+  @Type(() => Number)
+  @IsInt()
   page?: number;
 
-  @ApiPropertyOptional({ description: '每页数量' })
+  @ApiPropertyOptional({ description: '每页数量', default: 20 })
   @IsOptional()
+  @Type(() => Number)
+  @IsInt()
   pageSize?: number;
 }
 
@@ -128,12 +133,16 @@ export class QueryMyC2cOrdersDto {
   @IsIn(['PENDING', 'MATCHED', 'PAID', 'COMPLETED', 'CANCELLED', 'EXPIRED'])
   status?: string;
 
-  @ApiPropertyOptional({ description: '页码' })
+  @ApiPropertyOptional({ description: '页码', default: 1 })
   @IsOptional()
+  @Type(() => Number)
+  @IsInt()
   page?: number;
 
-  @ApiPropertyOptional({ description: '每页数量' })
+  @ApiPropertyOptional({ description: '每页数量', default: 20 })
   @IsOptional()
+  @Type(() => Number)
+  @IsInt()
   pageSize?: number;
 }