fix(multiplayer): reconcile authoritative initial connections

Game/scripts/server_control.gd

@@ -6,6 +6,7 @@ extends Node
 # controlled termination. Direct/community servers do not start this node.
 
 signal drain_requested
+signal initial_connect_ready
 
 var _listener := TCPServer.new()
 var _peers: Array = []
@@ -87,6 +88,18 @@ func _respond(peer: StreamPeerTCP, request: String) -> void:
 			drain_requested.emit()
 			status = 202
 			reason = "Accepted"
+	elif method == "POST" and path == "/initial-connect-ready":
+		var supplied := ""
+		for line in lines:
+			if line.begins_with("Authorization: Bearer "):
+				supplied = line.substr("Authorization: Bearer ".length())
+		if _drain_token.is_empty() or not _constant_time_equal(supplied, _drain_token):
+			status = 401
+			reason = "Unauthorized"
+		else:
+			initial_connect_ready.emit()
+			status = 202
+			reason = "Accepted"
 	else:
 		status = 405 if method in ["GET", "POST"] else 400
 		reason = "Method Not Allowed" if status == 405 else "Bad Request"