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/main/java/com/cameleer3/server/app/storage/ClickHouseMetricsQueryStore.java

@@ -0,0 +1,66 @@
+package com.cameleer3.server.app.storage;
+
+import com.cameleer3.server.core.storage.MetricsQueryStore;
+import com.cameleer3.server.core.storage.model.MetricTimeSeries;
+import org.springframework.jdbc.core.JdbcTemplate;
+
+import java.time.Instant;
+import java.util.*;
+
+public class ClickHouseMetricsQueryStore implements MetricsQueryStore {
+
+    private final JdbcTemplate jdbc;
+
+    public ClickHouseMetricsQueryStore(JdbcTemplate jdbc) {
+        this.jdbc = jdbc;
+    }
+
+    @Override
+    public Map<String, List<MetricTimeSeries.Bucket>> queryTimeSeries(
+            String agentId, List<String> metricNames,
+            Instant from, Instant to, int buckets) {
+
+        long intervalSeconds = Math.max(60,
+                (to.getEpochSecond() - from.getEpochSecond()) / Math.max(buckets, 1));
+
+        Map<String, List<MetricTimeSeries.Bucket>> result = new LinkedHashMap<>();
+        for (String name : metricNames) {
+            result.put(name.trim(), new ArrayList<>());
+        }
+
+        String[] namesArray = metricNames.stream().map(String::trim).toArray(String[]::new);
+
+        // ClickHouse JDBC doesn't support array params with IN (?).
+        // Build the IN clause with properly escaped values.
+        StringBuilder inClause = new StringBuilder();
+        for (int i = 0; i < namesArray.length; i++) {
+            if (i > 0) inClause.append(", ");
+            inClause.append("'").append(namesArray[i].replace("'", "\\'")).append("'");
+        }
+
+        String finalSql = """
+            SELECT toStartOfInterval(collected_at, INTERVAL %d SECOND) AS bucket,
+                   metric_name,
+                   avg(metric_value) AS avg_value
+            FROM agent_metrics
+            WHERE agent_id = ?
+              AND collected_at >= ?
+              AND collected_at < ?
+              AND metric_name IN (%s)
+            GROUP BY bucket, metric_name
+            ORDER BY bucket
+            """.formatted(intervalSeconds, inClause);
+
+        jdbc.query(finalSql, rs -> {
+            String metricName = rs.getString("metric_name");
+            Instant bucket = rs.getTimestamp("bucket").toInstant();
+            double value = rs.getDouble("avg_value");
+            result.computeIfAbsent(metricName, k -> new ArrayList<>())
+                    .add(new MetricTimeSeries.Bucket(bucket, value));
+        }, agentId,
+           java.sql.Timestamp.from(from),
+           java.sql.Timestamp.from(to));
+
+        return result;
+    }
+}