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);
+    }
+}

cameleer3-server-app/src/test/java/com/cameleer3/server/app/storage/ClickHouseMetricsStoreTest.java

@@ -0,0 +1,108 @@
+package com.cameleer3.server.app.storage;
+
+import com.cameleer3.server.core.storage.model.MetricsSnapshot;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.springframework.jdbc.core.JdbcTemplate;
+import org.testcontainers.clickhouse.ClickHouseContainer;
+import org.testcontainers.junit.jupiter.Container;
+import org.testcontainers.junit.jupiter.Testcontainers;
+
+import com.zaxxer.hikari.HikariDataSource;
+
+import java.time.Instant;
+import java.util.List;
+import java.util.Map;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+@Testcontainers
+class ClickHouseMetricsStoreTest {
+
+    @Container
+    static final ClickHouseContainer clickhouse =
+            new ClickHouseContainer("clickhouse/clickhouse-server:24.12");
+
+    private JdbcTemplate jdbc;
+    private ClickHouseMetricsStore store;
+
+    @BeforeEach
+    void setUp() {
+        HikariDataSource ds = new HikariDataSource();
+        ds.setJdbcUrl(clickhouse.getJdbcUrl());
+        ds.setUsername(clickhouse.getUsername());
+        ds.setPassword(clickhouse.getPassword());
+
+        jdbc = new JdbcTemplate(ds);
+
+        jdbc.execute("""
+            CREATE TABLE IF NOT EXISTS agent_metrics (
+                tenant_id          LowCardinality(String) DEFAULT 'default',
+                collected_at       DateTime64(3),
+                agent_id           LowCardinality(String),
+                metric_name        LowCardinality(String),
+                metric_value       Float64,
+                tags               Map(String, String) DEFAULT map(),
+                server_received_at DateTime64(3) DEFAULT now64(3)
+            )
+            ENGINE = MergeTree()
+            ORDER BY (tenant_id, agent_id, metric_name, collected_at)
+            """);
+
+        jdbc.execute("TRUNCATE TABLE agent_metrics");
+
+        store = new ClickHouseMetricsStore(jdbc);
+    }
+
+    @Test
+    void insertBatch_writesMetricsToClickHouse() {
+        List<MetricsSnapshot> batch = List.of(
+                new MetricsSnapshot("agent-1", Instant.parse("2026-03-31T10:00:00Z"),
+                        "cpu.usage", 75.5, Map.of("host", "server-1")),
+                new MetricsSnapshot("agent-1", Instant.parse("2026-03-31T10:00:01Z"),
+                        "memory.free", 1024.0, null)
+        );
+
+        store.insertBatch(batch);
+
+        Integer count = jdbc.queryForObject(
+                "SELECT count() FROM agent_metrics WHERE agent_id = 'agent-1'",
+                Integer.class);
+        assertThat(count).isEqualTo(2);
+    }
+
+    @Test
+    void insertBatch_storesTags() {
+        store.insertBatch(List.of(
+                new MetricsSnapshot("agent-2", Instant.parse("2026-03-31T10:00:00Z"),
+                        "disk.used", 500.0, Map.of("mount", "/data", "fs", "ext4"))
+        ));
+
+        // Just verify we can read back the row with tags
+        Integer count = jdbc.queryForObject(
+                "SELECT count() FROM agent_metrics WHERE agent_id = 'agent-2'",
+                Integer.class);
+        assertThat(count).isEqualTo(1);
+    }
+
+    @Test
+    void insertBatch_emptyList_doesNothing() {
+        store.insertBatch(List.of());
+
+        Integer count = jdbc.queryForObject("SELECT count() FROM agent_metrics", Integer.class);
+        assertThat(count).isEqualTo(0);
+    }
+
+    @Test
+    void insertBatch_nullTags_defaultsToEmptyMap() {
+        store.insertBatch(List.of(
+                new MetricsSnapshot("agent-3", Instant.parse("2026-03-31T10:00:00Z"),
+                        "cpu.usage", 50.0, null)
+        ));
+
+        Integer count = jdbc.queryForObject(
+                "SELECT count() FROM agent_metrics WHERE agent_id = 'agent-3'",
+                Integer.class);
+        assertThat(count).isEqualTo(1);
+    }
+}