POST /auth
Content-Type: application/json

{
  "user_id": "your_user_id",
  "secret": "your_secret"
}

{
    "access_token": "eyJhbGciOiJSUzI1...",
    "type": "Bearer",
    "expiration_time": 1200
}

function getValidToken():
  if cached_token exists AND now < cached_expires_at - safety_window:
      return cached_token

  lock(token_refresh_lock):
      # Double-check after acquiring lock
      if cached_token exists AND now < cached_expires_at - safety_window:
          return cached_token

      response = requestNewToken()
      cached_token = response.access_token
      cached_expires_at = now + response.expires_in
      return cached_token