fix(monitoring): correct Kafka metrics and alert rules

- Why:
  - dashboard showed offset as throughput and produced misleading values
  - kafka-exporter metric/label naming was inconsistent across alerts/docs
  - consumer-group-missing alert was noisy for demo runs
- What:
  - switch throughput panel to rate(kafka_topic_partition_current_offset[5m]) aggregated by topic and exclude __* topics
  - align lag metric/labels to kafka_consumergroup_lag + consumergroup
  - remove Kafka Consumer Group Missing alert from provisioning
  - pin kafka-exporter image to v1.9.0 and update OPERATIONS.md checks
- Check:
  - airflow dags list-import-errors -> No data found
  - Prometheus targets: clickhouse up, kafka up
  - PromQL kafka_consumergroup_lag returns series
  - Grafana dashboards provisioning reload returns success

docker-compose.yml

@@ -232,7 +232,7 @@ services:
 
   # Kafka Exporter для мониторинга через Prometheus
   kafka-exporter:
-    image: danielqsj/kafka-exporter:latest
+    image: danielqsj/kafka-exporter:v1.9.0
     command: ["--kafka.server=kafka:29092"]
     ports:
       - "9308:9308"