Periodically sweeps users whose is_online flag is true but whose last_seen_at timestamp is older than a configurable threshold.

This is a safety net for node crashes or ungraceful disconnects where the UserChannel.terminate/2 callback never fires. Without this, users would remain marked as online indefinitely.

Configuration

config :game_server_core, GameServer.Accounts.StalePresenceSweeper,
  interval_ms: 120_000,       # how often to run the sweep (default 2 min)
  stale_threshold_s: 300,     # mark offline if last_seen > 5 min ago
  enabled: true               # set false to disable the sweep entirely