fix: make conntrack flush best-effort when conntrack binary is missing

CI environments may not have the conntrack binary installed. The flush
is a performance optimization, not a correctness requirement. Log and
continue instead of returning an error.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Frando

patchbay/src/balancer.rs

@@ -666,16 +666,20 @@ impl crate::router::Router {
                         &port.to_string(),
                     ])
                     .status()
-                    .await
-                    .context("spawn conntrack -D")?;
-                // conntrack -D returns non-zero if no entries matched, which is fine.
-                debug!(
-                    vip = %vip,
-                    port = %port,
-                    proto = %proto,
-                    success = status.success(),
-                    "balancer: conntrack flush"
-                );
+                    .await;
+                // conntrack may not be installed or may find no matching entries.
+                // Both are fine. Log and continue.
+                match status {
+                    Ok(s) => debug!(
+                        vip = %vip, port = %port, proto = %proto,
+                        success = s.success(),
+                        "balancer: conntrack flush"
+                    ),
+                    Err(e) => debug!(
+                        error = %e,
+                        "balancer: conntrack not available, skipping flush"
+                    ),
+                }
             }
             Ok(())
         };