feat: add ClickHouseMetricsStore with batch insert

TDD implementation of MetricsStore backed by ClickHouse. Uses native
Map(String,String) column type (no JSON cast), relies on ClickHouse
DEFAULT for server_received_at, and handles null tags by substituting
an empty HashMap. All 4 Testcontainers tests pass.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cameleer3-server-app/src/main/java/com/cameleer3/server/app/storage/ClickHouseMetricsStore.java

@@ -0,0 +1,41 @@
+package com.cameleer3.server.app.storage;
+
+import com.cameleer3.server.core.storage.MetricsStore;
+import com.cameleer3.server.core.storage.model.MetricsSnapshot;
+import org.springframework.jdbc.core.JdbcTemplate;
+
+import java.sql.Timestamp;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+public class ClickHouseMetricsStore implements MetricsStore {
+
+    private final JdbcTemplate jdbc;
+
+    public ClickHouseMetricsStore(JdbcTemplate jdbc) {
+        this.jdbc = jdbc;
+    }
+
+    @Override
+    public void insertBatch(List<MetricsSnapshot> snapshots) {
+        if (snapshots.isEmpty()) return;
+
+        jdbc.batchUpdate("""
+                INSERT INTO agent_metrics (agent_id, metric_name, metric_value, tags, collected_at)
+                VALUES (?, ?, ?, ?, ?)
+                """,
+                snapshots.stream().map(s -> new Object[]{
+                        s.agentId(),
+                        s.metricName(),
+                        s.metricValue(),
+                        tagsToClickHouseMap(s.tags()),
+                        Timestamp.from(s.collectedAt())
+                }).toList());
+    }
+
+    private Map<String, String> tagsToClickHouseMap(Map<String, String> tags) {
+        if (tags == null || tags.isEmpty()) return new HashMap<>();
+        return new HashMap<>(tags);
+    }
+}