Make stats endpoint respect selected time window instead of hardcoded last hour

P99 latency and active count now use the same from/to parameters as the
timeseries sparklines, so all stat cards are consistent with the user's
selected time range.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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

@@ -70,8 +70,11 @@ public class SearchController {
 
     @GetMapping("/stats")
     @Operation(summary = "Aggregate execution stats (P99 latency, active count)")
-    public ResponseEntity<ExecutionStats> stats() {
-        return ResponseEntity.ok(searchService.stats());
+    public ResponseEntity<ExecutionStats> stats(
+            @RequestParam Instant from,
+            @RequestParam(required = false) Instant to) {
+        Instant end = to != null ? to : Instant.now();
+        return ResponseEntity.ok(searchService.stats(from, end));
     }
 
     @GetMapping("/stats/timeseries")