fix(gateway): remove duplicate /leaderboard route from referral-service

The /api/v1/leaderboard route was duplicated in both referral-service
and leaderboard-service. Since leaderboard-service is the dedicated
ranking service with more features (daily/weekly/monthly rankings,
virtual accounts, snapshots), all leaderboard requests should be
routed to leaderboard-service.

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

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

backend/api-gateway/kong.yml

@@ -108,10 +108,7 @@ services:
         paths:
           - /api/v1/referrals
         strip_path: false
-      - name: referral-leaderboard
-        paths:
-          - /api/v1/leaderboard
-        strip_path: false
+      # 注意: /api/v1/leaderboard 已移至 leaderboard-service
       - name: referral-team-statistics
         paths:
           - /api/v1/team-statistics