fix(leaderboard): add REFERRAL_SERVICE_URL to docker-compose

The leaderboard-service needs to connect to referral-service for
team statistics data. Without this environment variable, it falls
back to localhost:3004 which fails inside Docker network.

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

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

backend/services/docker-compose.yml

@@ -459,6 +459,9 @@ services:
       - KAFKA_BROKERS=kafka:29092
       - KAFKA_CLIENT_ID=leaderboard-service
       - KAFKA_GROUP_ID=leaderboard-service-group
+      # External Service URLs
+      - REFERRAL_SERVICE_URL=http://rwa-referral-service:3004
+      - IDENTITY_SERVICE_URL=http://rwa-identity-service:3000
     depends_on:
       postgres:
         condition: service_healthy
@@ -466,6 +469,8 @@ services:
         condition: service_healthy
       kafka:
         condition: service_started
+      referral-service:
+        condition: service_healthy
     healthcheck:
       test: ["CMD", "curl", "-f", "http://localhost:3007/api/health"]
       interval: 30s