fix(pre-planting): 无推荐关系用户购买预种时 404 导致整笔交易失败

问题：PrePlantingReferralClient.getReferralChain() 生产环境遇到 404
（用户无推荐人）时直接 throw error，导致整个购买事务回滚，
无推荐关系的用户（测试账号、直接注册用户）完全无法购买预种份额。

修复：AxiosError status === 404 时返回 { directReferrer: null }，
与"直接注册、无推荐人"的正常业务场景对齐，不阻断购买流程。

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/services/planting-service/src/pre-planting/infrastructure/external/pre-planting-referral.client.ts

@@ -37,14 +37,18 @@ export class PrePlantingReferralClient {
       );
       return response.data;
     } catch (error) {
+      // 404 = 用户无推荐关系，属于正常情况（如测试账号、直接注册用户）
+      // 返回空推荐链而非抛错，避免阻断购买流程
+      if (error?.response?.status === 404) {
+        this.logger.warn(
+          `No referral chain found for ${accountSequence}, proceeding without referrer`,
+        );
+        return { accountSequence, directReferrer: null };
+      }
       this.logger.error(
         `Failed to get referral chain for ${accountSequence}`,
         error,
       );
-      if (this.configService.get('NODE_ENV') === 'development') {
-        this.logger.warn('Development mode: returning empty referral chain');
-        return { accountSequence, directReferrer: null };
-      }
       throw error;
     }
   }