Revert "feat: Track user traffic per node (server_id)"

app/Models/StatUser.php

@@ -9,7 +9,6 @@ use Illuminate\Database\Eloquent\Model;
  *
  * @property int $id
  * @property int $user_id 用户ID
- * @property int|null $server_id 节点ID (nullable for legacy data)
  * @property int $u 上行流量
  * @property int $d 下行流量
  * @property int $record_at 记录时间
@@ -26,12 +25,4 @@ class StatUser extends Model
         'created_at' => 'timestamp',
         'updated_at' => 'timestamp'
     ];
-
-    /**
-     * Get the server that this traffic stat belongs to
-     */
-    public function server()
-    {
-        return $this->belongsTo(Server::class, 'server_id');
-    }
 }