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>

ui/src/api/schema.d.ts

@@ -97,6 +97,12 @@ export interface paths {
   };
   '/search/stats': {
     get: {
+      parameters: {
+        query: {
+          from: string;
+          to?: string;
+        };
+      };
       responses: {
         200: {
           content: {