feat(api): add api_uptime_seconds metric, replace web-gateway panels in cockpit

- Add api_uptime_seconds gauge to API service (5s update interval),
  matching the pattern used by trading/backtesting/ml-training services
- Remove obsolete Web Gateway Uptime and Active WebSocket Connections
  panels from cockpit (web-gateway is not deployed)
- Add API Gateway Uptime (api_uptime_seconds) and API Auth Rate
  (api_auth_requests_total) panels in their place

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

services/api/src/main.rs

@@ -543,6 +543,22 @@ async fn main() -> Result<()> {
     .map_err(|e| anyhow::anyhow!("Failed to register service info: {}", e))?;
     service_info.set(1.0);
 
+    // Add uptime gauge (matches pattern of other services)
+    let uptime_gauge = register_gauge_with_registry!(
+        Opts::new("api_uptime_seconds", "API service uptime in seconds"),
+        gateway_metrics.registry().as_ref()
+    )
+    .map_err(|e| anyhow::anyhow!("Failed to register uptime gauge: {}", e))?;
+    let start_instant = std::time::Instant::now();
+    let uptime_clone = uptime_gauge.clone();
+    tokio::spawn(async move {
+        let mut tick = tokio::time::interval(Duration::from_secs(5));
+        loop {
+            tick.tick().await;
+            uptime_clone.set(start_instant.elapsed().as_secs_f64());
+        }
+    });
+
     let metrics_registry = gateway_metrics.registry();
 
     tokio::spawn(async move {