fix(referral): convert BigInt to string for JSON serialization

BigInt cannot be serialized by JSON.stringify, convert to string
before returning from the API endpoint.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

backend/services/referral-service/src/api/controllers/referral.controller.ts

@@ -123,7 +123,7 @@ export class ReferralController {
   })
   async getReferralChainForReward(
     @Param('userId') userId: string,
-  ): Promise<{ ancestors: Array<{ userId: bigint; hasPlanted: boolean }> }> {
+  ): Promise<{ ancestors: Array<{ userId: string; hasPlanted: boolean }> }> {
     const userIdBigInt = BigInt(userId);
 
     // 获取用户的推荐关系
@@ -140,7 +140,7 @@ export class ReferralController {
     return {
       ancestors: [
         {
-          userId: referrerId,
+          userId: referrerId.toString(),
           hasPlanted,
         },
       ],