Add stat card sparkline graphs with timeseries backend endpoint

New /search/stats/timeseries endpoint returns bucketed counts/metrics
over a time window using ClickHouse toStartOfInterval(). Frontend
Sparkline component renders SVG polyline + gradient fill on each
stat card, driven by a useStatsTimeseries query hook.

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

cameleer3-server-core/src/main/java/com/cameleer3/server/core/search/SearchEngine.java

@@ -31,4 +31,14 @@ public interface SearchEngine {
      * @return execution stats
      */
     ExecutionStats stats();
+
+    /**
+     * Compute bucketed time-series stats over a time window.
+     *
+     * @param from start of the time window
+     * @param to end of the time window
+     * @param bucketCount number of buckets to divide the window into
+     * @return bucketed stats
+     */
+    StatsTimeseries timeseries(java.time.Instant from, java.time.Instant to, int bucketCount);
 }

cameleer3-server-core/src/main/java/com/cameleer3/server/core/search/SearchService.java

@@ -35,4 +35,11 @@ public class SearchService {
     public ExecutionStats stats() {
         return engine.stats();
     }
+
+    /**
+     * Compute bucketed time-series stats over a time window.
+     */
+    public StatsTimeseries timeseries(java.time.Instant from, java.time.Instant to, int bucketCount) {
+        return engine.timeseries(from, to, bucketCount);
+    }
 }

cameleer3-server-core/src/main/java/com/cameleer3/server/core/search/StatsTimeseries.java

@@ -0,0 +1,17 @@
+package com.cameleer3.server.core.search;
+
+import java.time.Instant;
+import java.util.List;
+
+public record StatsTimeseries(
+    List<TimeseriesBucket> buckets
+) {
+    public record TimeseriesBucket(
+        Instant time,
+        long totalCount,
+        long failedCount,
+        long avgDurationMs,
+        long p99DurationMs,
+        long activeCount
+    ) {}
+}