feat: enable Prometheus metrics for ClickHouse and tenant servers

ClickHouse: enable built-in Prometheus exporter at :9363/metrics via
config.d/prometheus.xml with metrics, events, and async_metrics.
Docker labels added for docker_sd_configs auto-discovery.

Tenant servers: add prometheus.scrape/path/port labels to provisioned
server containers pointing to /api/v1/prometheus:8081.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/main/java/net/siegeln/cameleer/saas/provisioning/DockerTenantProvisioner.java

@@ -190,6 +190,9 @@ public class DockerTenantProvisioner implements TenantProvisioner {
         labels.put("traefik.docker.network", props.traefikNetwork());
         labels.put("cameleer.tenant", slug);
         labels.put("cameleer.role", "server");
+        labels.put("prometheus.scrape", "true");
+        labels.put("prometheus.path", "/api/v1/prometheus");
+        labels.put("prometheus.port", "8081");
 
         var env = new java.util.ArrayList<>(List.of(
             "SPRING_DATASOURCE_URL=" + props.datasourceUrl(),