feature: player management plugin sync for community servers

[lukepolo]

Api side of the new Player Management plugin, so panel sanctions are enforced live on community servers.

• GET /sanctions/server/:id (unused) becomes POST: the plugin sends its players and gets their active sanctions; the call doubles as its heartbeat
• Heartbeat stored in new servers.player_management_{version,runtime,seen_at}
• Sanctioning on a community server sends RCON player_management_refresh and reports whether it took effect
• Community CS2 dedicated servers on a node get INSTALL_PLAYER_MANAGEMENT_PLUGIN=true
• Moderators can select servers.game_server_node_id and offline_at, which the dedicated-server page needs