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/openapi.json

@@ -664,6 +664,26 @@
         ],
         "summary": "Aggregate execution stats (P99 latency, active count)",
         "operationId": "stats",
+        "parameters": [
+          {
+            "name": "from",
+            "in": "query",
+            "required": true,
+            "schema": {
+              "type": "string",
+              "format": "date-time"
+            }
+          },
+          {
+            "name": "to",
+            "in": "query",
+            "required": false,
+            "schema": {
+              "type": "string",
+              "format": "date-time"
+            }
+          }
+        ],
         "responses": {
           "200": {
             "description": "OK",