Expose process PID in API and UI

Include the process PID in the API info payload and surface it in the frontend. apiHandler now adds app.Info["pid"] = os.Getpid(); openapi.yaml documents the new pid property as an integer with an example; www/index.html is updated to show pid alongside version and config. This aids debugging and identifying the running process.

internal/api/api.go

@@ -238,6 +238,7 @@ var mu sync.Mutex
 func apiHandler(w http.ResponseWriter, r *http.Request) {
 	mu.Lock()
 	app.Info["host"] = r.Host
+	app.Info["pid"] = os.Getpid()
 	mu.Unlock()
 
 	ResponseJSON(w, app.Info)