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-app/src/main/java/com/cameleer3/server/app/controller/SearchController.java

@@ -5,6 +5,7 @@ import com.cameleer3.server.core.search.ExecutionSummary;
 import com.cameleer3.server.core.search.SearchRequest;
 import com.cameleer3.server.core.search.SearchResult;
 import com.cameleer3.server.core.search.SearchService;
+import com.cameleer3.server.core.search.StatsTimeseries;
 import io.swagger.v3.oas.annotations.Operation;
 import io.swagger.v3.oas.annotations.tags.Tag;
 import org.springframework.http.ResponseEntity;
@@ -72,4 +73,14 @@ public class SearchController {
     public ResponseEntity<ExecutionStats> stats() {
         return ResponseEntity.ok(searchService.stats());
     }
+
+    @GetMapping("/stats/timeseries")
+    @Operation(summary = "Bucketed time-series stats over a time window")
+    public ResponseEntity<StatsTimeseries> timeseries(
+            @RequestParam Instant from,
+            @RequestParam(required = false) Instant to,
+            @RequestParam(defaultValue = "24") int buckets) {
+        Instant end = to != null ? to : Instant.now();
+        return ResponseEntity.ok(searchService.timeseries(from, end, buckets));
+    }
 }