refactor(stats): replace separate successful retrievals and error rate with combined retrieval success rate

- Simplify system stats display by using a single "Retrieval Success Rate" metric instead of two separate fields for successful retrievals and error rate
- Improves readability and consolidates related performance data into one percentage-based indicator

app.py

@@ -352,8 +352,7 @@ def get_system_stats() -> str:
 
     # Recent Activity
     output += "### 📈 Recent Activity\n\n"
-    output += f"**Successful Retrievals:** {metrics['system_health']['successful_retrievals'] if 'successful_retrievals' in metrics['system_health'] else 'N/A'}\n\n"
-    output += f"**Error Rate:** {metrics['system_health']['error_rate']:.1% if 'error_rate' in metrics['system_health'] else 'N/A'}\n\n"
+    output += f"**Retrieval Success Rate:** {metrics['system_health']['retrieval_success_rate']:.1% if 'retrieval_success_rate' in metrics['system_health'] else 'N/A'}\n\n"
 
     return output