fix: reload sentiment cycle history after backfill

backfillData() only refreshed system settings after a successful backfill,
leaving state.sentimentHistory/sentimentHistoryKey populated with the stale
pre-backfill cache, so an already-open sentiment cycle page kept showing the
old one-day history.

After a successful backfill, clear both cache fields and, when the sentiment
cycle view is active, force loadSentimentHistory(true) so the history renders
immediately. Add a frontend contract test covering the refresh path.

Co-authored-by: multica-agent <github@multica.ai>

app/frontend/shared/admin.js

@@ -8,6 +8,11 @@ async function backfillData() {
       end_date: document.querySelector("#backfillEnd").value,
     });
     showToast(`历史回补完成，共处理 ${payload.results.length} 个工作日`);
+    state.sentimentHistory = null;
+    state.sentimentHistoryKey = "";
+    if (state.activeView === "sentimentCycleView") {
+      await loadSentimentHistory(true);
+    }
     await openAdminSettings(true);
   } catch (error) {
     showToast(error.message);