feat: optional NPM proxy per stage

Add enable_proxy boolean to stages (default true). When disabled,
the deployer skips NPM proxy host creation — useful for internal
services, workers, or externally-routed containers. UI shows
toggle in Add Stage form and "No Proxy" badge on stage header.

internal/store/models.go

@@ -23,6 +23,7 @@ type Stage struct {
 	AutoDeploy   bool   `json:"auto_deploy"`
 	MaxInstances int    `json:"max_instances"`
 	Confirm      bool   `json:"confirm"`
+	EnableProxy  bool   `json:"enable_proxy"`
 	PromoteFrom  string `json:"promote_from"`
 	Subdomain    string `json:"subdomain"`
 	CreatedAt    string `json:"created_at"`