feat: add ClickHouseMetricsQueryStore with time-bucketed queries

Implements MetricsQueryStore using ClickHouse toStartOfInterval() for
time-bucketed aggregation queries; verified with 4 Testcontainers tests.

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

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

@@ -0,0 +1,114 @@
+package com.cameleer3.server.app.storage;
+
+import com.cameleer3.server.core.storage.model.MetricTimeSeries;
+import com.zaxxer.hikari.HikariDataSource;
+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 java.time.Instant;
+import java.util.List;
+import java.util.Map;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+@Testcontainers
+class ClickHouseMetricsQueryStoreTest {
+
+    @Container
+    static final ClickHouseContainer clickhouse =
+            new ClickHouseContainer("clickhouse/clickhouse-server:24.12");
+
+    private JdbcTemplate jdbc;
+    private ClickHouseMetricsQueryStore queryStore;
+
+    @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");
+
+        // Seed test data: 6 data points across 1 hour for two metrics
+        Instant base = Instant.parse("2026-03-31T10:00:00Z");
+        for (int i = 0; i < 6; i++) {
+            Instant ts = base.plusSeconds(i * 600); // every 10 minutes
+            jdbc.update("INSERT INTO agent_metrics (agent_id, metric_name, metric_value, collected_at) VALUES (?, ?, ?, ?)",
+                    "agent-1", "cpu.usage", 50.0 + i * 5, java.sql.Timestamp.from(ts));
+            jdbc.update("INSERT INTO agent_metrics (agent_id, metric_name, metric_value, collected_at) VALUES (?, ?, ?, ?)",
+                    "agent-1", "memory.free", 1000.0 - i * 100, java.sql.Timestamp.from(ts));
+        }
+
+        queryStore = new ClickHouseMetricsQueryStore(jdbc);
+    }
+
+    @Test
+    void queryTimeSeries_returnsDataGroupedByMetric() {
+        Instant from = Instant.parse("2026-03-31T10:00:00Z");
+        Instant to = Instant.parse("2026-03-31T11:00:00Z");
+
+        Map<String, List<MetricTimeSeries.Bucket>> result =
+                queryStore.queryTimeSeries("agent-1", List.of("cpu.usage", "memory.free"), from, to, 6);
+
+        assertThat(result).containsKeys("cpu.usage", "memory.free");
+        assertThat(result.get("cpu.usage")).isNotEmpty();
+        assertThat(result.get("memory.free")).isNotEmpty();
+    }
+
+    @Test
+    void queryTimeSeries_bucketsAverageCorrectly() {
+        Instant from = Instant.parse("2026-03-31T10:00:00Z");
+        Instant to = Instant.parse("2026-03-31T11:00:00Z");
+
+        // 1 bucket for the entire hour = average of all 6 values
+        Map<String, List<MetricTimeSeries.Bucket>> result =
+                queryStore.queryTimeSeries("agent-1", List.of("cpu.usage"), from, to, 1);
+
+        assertThat(result.get("cpu.usage")).hasSize(1);
+        // Values: 50, 55, 60, 65, 70, 75 → avg = 62.5
+        assertThat(result.get("cpu.usage").get(0).value()).isCloseTo(62.5, org.assertj.core.data.Offset.offset(0.1));
+    }
+
+    @Test
+    void queryTimeSeries_noData_returnsEmptyLists() {
+        Instant from = Instant.parse("2025-01-01T00:00:00Z");
+        Instant to = Instant.parse("2025-01-01T01:00:00Z");
+
+        Map<String, List<MetricTimeSeries.Bucket>> result =
+                queryStore.queryTimeSeries("agent-1", List.of("cpu.usage"), from, to, 6);
+
+        assertThat(result.get("cpu.usage")).isEmpty();
+    }
+
+    @Test
+    void queryTimeSeries_unknownAgent_returnsEmpty() {
+        Instant from = Instant.parse("2026-03-31T10:00:00Z");
+        Instant to = Instant.parse("2026-03-31T11:00:00Z");
+
+        Map<String, List<MetricTimeSeries.Bucket>> result =
+                queryStore.queryTimeSeries("nonexistent", List.of("cpu.usage"), from, to, 6);
+
+        assertThat(result.get("cpu.usage")).isEmpty();
+    }
+}