feat(monitoring): add Airflow monitoring via statsd-exporter

- Add statsd-exporter service to docker-compose.yml (prom/statsd-exporter:v0.27.1)
- Add StatsD env vars to airflow-default-env for metrics export
- Add airflow job to prometheus.yml scrape configs
- Add Airflow Overview dashboard (Grafana provisioning)
- Add Airflow alert rules: scheduler down, queue backlog, failures, parse time
- Add configs/statsd_mapping.yml for StatsD → Prometheus conversion
- Use Prometheus naming convention (_total for counters, _seconds for timers)
- Add monitoring plan at plans/monitoring_airflow_plan.md
- Update OPERATIONS.md and Makefile for airflow monitoring

Tested: all 3 jobs (airflow, clickhouse, kafka) showing UP in Prometheus,
metrics flowing (dagbag_size=3, executor slots, heartbeats with _total suffix),
all 4 alert rules loaded in Grafana

configs/prometheus.yml

@@ -18,4 +18,13 @@ scrape_configs:
       - targets: ["kafka-exporter:9308"]
         labels:
           instance: Kafka-1
+    honor_labels: true
+
+  # Airflow metrics via statsd-exporter (StatsD → Prometheus)
+  - job_name: "airflow"
+    metrics_path: "/metrics"
+    static_configs:
+      - targets: ["statsd-exporter:9102"]
+        labels:
+          instance: Airflow-1
     honor_labels: true