feat(server): persist server self-metrics into ClickHouse

Snapshot the full Micrometer registry (cameleer business metrics, alerting
metrics, and Spring Boot Actuator defaults) every 60s into a new
server_metrics table so server health survives restarts without an external
Prometheus. Includes a dashboard-builder reference for the SaaS team.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cameleer-server-core/src/main/java/com/cameleer/server/core/storage/ServerMetricsStore.java

@@ -0,0 +1,16 @@
+package com.cameleer.server.core.storage;
+
+import com.cameleer.server.core.storage.model.ServerMetricSample;
+
+import java.util.List;
+
+/**
+ * Sink for periodic snapshots of the server's own Micrometer meter registry.
+ * Implementations persist the samples (e.g. to ClickHouse) so server
+ * self-metrics survive restarts and can be queried historically without an
+ * external Prometheus.
+ */
+public interface ServerMetricsStore {
+
+    void insertBatch(List<ServerMetricSample> samples);
+}

cameleer-server-core/src/main/java/com/cameleer/server/core/storage/model/ServerMetricSample.java

@@ -0,0 +1,23 @@
+package com.cameleer.server.core.storage.model;
+
+import java.time.Instant;
+import java.util.Map;
+
+/**
+ * A single sample of the server's own Micrometer registry, captured by a
+ * scheduled snapshot and destined for the ClickHouse {@code server_metrics}
+ * table. One {@code ServerMetricSample} per Micrometer {@code Measurement},
+ * so Timers and DistributionSummaries produce multiple samples per tick
+ * (distinguished by {@link #statistic()}).
+ */
+public record ServerMetricSample(
+        String tenantId,
+        Instant collectedAt,
+        String serverInstanceId,
+        String metricName,
+        String metricType,
+        String statistic,
+        double value,
+        Map<String, String> tags
+) {
+}