feat(monitoring): add Grafana dashboard for ClickHouse

- Add Prometheus datasource provisioning config
- Add Grafana dashboard provider configuration
- Add ClickHouse Overview dashboard (JSON)
- Update README.md with monitoring section and updated stack badge
- Update docs/OPERATIONS.md with monitoring runbook

Dashboard includes:
- System Health: CPU, Memory Resident, Memory Code
- Query Performance: queries/sec, active queries, failed queries
- MergeTree Storage: parts count, merge rate

Verified via Context7: ClickHouse Prometheus metrics use ClickHouseAsyncMetrics_*,
ClickHouseMetrics_*, ClickHouseProfileEvents_* prefixes in v25.1.

Access:
- Grafana: http://localhost:3000 (admin/admin)
- Prometheus: http://localhost:9090

configs/grafana/provisioning/dashboards/dashboard.yml

@@ -0,0 +1,16 @@
+# -----------------------------------------------------------------------------
+# Dashboard Provider: автоматическая загрузка дашбордов из файлов
+# Назначение: Grafana автоматически импортирует дашборды из указанной папки
+# -----------------------------------------------------------------------------
+
+apiVersion: 1
+
+providers:
+  - name: "default"
+    orgId: 1
+    folder: ""
+    type: file
+    disableDeletion: false
+    editable: true
+    options:
+      path: /etc/grafana/provisioning/dashboards

configs/grafana/provisioning/datasources/prometheus.yml

@@ -0,0 +1,17 @@
+# -----------------------------------------------------------------------------
+# DataSource: Prometheus для Grafana
+# Назначение: автоматическая настройка подключения к Prometheus
+# -----------------------------------------------------------------------------
+
+apiVersion: 1
+
+datasources:
+  - name: Prometheus
+    type: prometheus
+    access: proxy
+    url: http://prometheus:9090
+    isDefault: true
+    editable: false
+    jsonData:
+      timeInterval: "15s"
+      httpMethod: POST