feat(monitoring): add Kafka monitoring via kafka-exporter

- Add kafka-exporter service to docker-compose.yml
- Add kafka job to prometheus.yml scrape configs
- Add Kafka Overview dashboard (Grafana provisioning)
- Add Kafka alert rules (broker down, consumer lag, etc.)
- Add make reload-monitoring command for easy updates
- Update OPERATIONS.md with TL;DR and troubleshooting

API verified via Context7:
- /danielqsj/kafka_exporter for exporter config
- /prometheus/docs for scrape_configs format

configs/prometheus.yml

@@ -1,6 +1,6 @@
 global:
   scrape_interval: 15s
-  
+
 scrape_configs:
   - job_name: "clickhouse"
     metrics_path: "/metrics"
@@ -8,4 +8,14 @@ scrape_configs:
       - targets: ["clickhouse:9126"]
         labels:
           instance: Clickhouse-1
+    honor_labels: true
+
+  # Kafka metrics via kafka-exporter
+  # Проверено через Context7: формат scrape_configs подтверждён (/prometheus/docs)
+  - job_name: "kafka"
+    metrics_path: "/metrics"
+    static_configs:
+      - targets: ["kafka-exporter:9308"]
+        labels:
+          instance: Kafka-1
     honor_labels: true