perf: reuse resolved count for reopen rate (#15122)

This improves the Captain overview by loading reporting metrics and FAQ
stats from separate endpoints. Range changes now refresh only the
metrics, while reopen-rate calculation reuses the resolved conversation
count to avoid redundant database queries.

## What changed

- Split Captain overview metrics and FAQ stats into separate APIs.
- Fetch FAQ stats independently from range-based metrics.
- Reuse resolved conversation totals when calculating reopen rate.
- Skip the reopen query when there are no resolved conversations.

app/javascript/dashboard/api/captain/assistant.js

@@ -26,13 +26,20 @@ class CaptainAssistant extends ApiClient {
     });
   }
 
-  getStats({ assistantId, range, signal }) {
+  getMetrics({ assistantId, range, signal }) {
     const requestConfig = {
       params: { range, timezone_offset: getTimezoneOffset() },
     };
     if (signal) requestConfig.signal = signal;
 
-    return axios.get(`${this.url}/${assistantId}/stats`, requestConfig);
+    return axios.get(`${this.url}/${assistantId}/metrics`, requestConfig);
+  }
+
+  getFaqStats({ assistantId, signal }) {
+    const requestConfig = {};
+    if (signal) requestConfig.signal = signal;
+
+    return axios.get(`${this.url}/${assistantId}/faq_stats`, requestConfig);
   }
 
   getSummary({ assistantId, range, stats }) {