fix(infra): remove double-dollar escape in Fluent Bit label_keys

In Terraform quoted strings $var is literal — only ${var} triggers
interpolation. The $$ was passing through as literal $$kube_* to
Fluent Bit, causing a record accessor syntax error on startup.

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

infrastructure/terraform/monitoring.tf

@@ -215,7 +215,7 @@ resource "helm_release" "fluent_bit" {
         # Static label keeps backward compat; dynamic labels let you filter
         # {namespace="finance"} or {app="api"} in Grafana/LogQL.
         "    Labels      job=fluent-bit\n",
-        "    label_keys  $$kube_namespace_name,$$kube_container_name\n",
+        "    label_keys  $kube_namespace_name,$kube_container_name\n",
       ])
     }
     tolerations = [