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>

ui/src/api/schema.d.ts

@@ -106,6 +106,24 @@ export interface paths {
       };
     };
   };
+  '/search/stats/timeseries': {
+    get: {
+      parameters: {
+        query: {
+          from: string;
+          to?: string;
+          buckets?: number;
+        };
+      };
+      responses: {
+        200: {
+          content: {
+            'application/json': StatsTimeseries;
+          };
+        };
+      };
+    };
+  };
   '/agents': {
     get: {
       parameters: {
@@ -197,6 +215,19 @@ export interface ExecutionStats {
   activeCount: number;
 }
 
+export interface StatsTimeseries {
+  buckets: TimeseriesBucket[];
+}
+
+export interface TimeseriesBucket {
+  time: string;
+  totalCount: number;
+  failedCount: number;
+  avgDurationMs: number;
+  p99DurationMs: number;
+  activeCount: number;
+}
+
 export interface AgentInstance {
   id: string;
   name: string;