fix: bump argocd route timeout past Envoy's 15s default

Envoy Gateway's per-route default timeout (15s, unset anywhere in this
cluster) was cutting the connection to argocd-server mid-Sync, surfacing
as 'Gateway Timeout' in the UI — confirmed even a single-resource sync
hit exactly ~15.0s every attempt, ruling out chart size/congestion.

BackendTrafficPolicy on the argocd HTTPRoute raises requestTimeout to 120s.

manifests/argocd/backend-traffic-policy.yaml

@@ -0,0 +1,13 @@
+apiVersion: gateway.envoyproxy.io/v1alpha1
+kind: BackendTrafficPolicy
+metadata:
+  name: argocd
+  namespace: argocd
+spec:
+  targetRefs:
+    - group: gateway.networking.k8s.io
+      kind: HTTPRoute
+      name: argocd
+  timeout:
+    http:
+      requestTimeout: 120s