[api] Threat server pubkey change as a crash

app/Models/Server.php

@@ -96,6 +96,9 @@ public function getJWTCustomClaims()
 
     public function heartbeat(string $ip, int $port, string $pubkey)
     {
+        if ($pubkey !== $this->pubkey)
+            $this->stop();
+
         $this->last_heartbeat_at = Carbon::now();
         $this->online = true;
         $this->ip = $ip;