feat(infra): add backend service URLs to api-gateway K8s deployment

Add BROKER_GATEWAY_SERVICE_URL, DATA_ACQUISITION_SERVICE_URL, and
ML_SERVICE_URL env vars so the gateway can connect to all backend services.

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

infra/k8s/services/api-gateway.yaml

@@ -78,6 +78,12 @@ spec:
               value: "http://ml-training-service:50053"
             - name: TRADING_AGENT_SERVICE_URL
               value: "http://trading-agent-service:50055"
+            - name: BROKER_GATEWAY_SERVICE_URL
+              value: "http://broker-gateway:50056"
+            - name: DATA_ACQUISITION_SERVICE_URL
+              value: "http://data-acquisition-service:50057"
+            - name: ML_SERVICE_URL
+              value: "http://trading-service:50051"
             - name: RUST_LOG
               value: info
             - name: OTEL_EXPORTER_OTLP_ENDPOINT