feat(CS): run the siege on ONE designated server (CastleSiegeServerId)

With multiple game servers each has its own map instances, so the siege ran
independently on all of them - a guild could win uncontested on an empty server's
Valley of Loren. Now the siege (tick/spawn/battle/registration) runs only on the
server whose Id == config.CastleSiegeServerId (AdminPanel-editable). Other servers
skip the siege and just mirror the shared castle owner from config so the hunting-map
gate + castle flag rewards still work everywhere. The Guardsman on non-siege servers
tells players which server to switch to.

src/GameLogic/CastleSiege/CastleSiegeGuardsmanTalkPlugIn.cs

@@ -43,6 +43,12 @@ public class CastleSiegeGuardsmanTalkPlugIn : IPlayerTalkToNpcPlugIn
             return;
         }
 
+        if (!CastleSiegeEventPlugIn.IsCastleSiegeServer(player.GameContext))
+        {
+            await ShowAsync(player, $"The Castle Siege runs on server {context.Configuration.CastleSiegeServerId}. Switch to that server to register and fight.").ConfigureAwait(false);
+            return;
+        }
+
         if (context.Phase != CastleSiegePhase.Registration)
         {
             await ShowAsync(player, "Castle Siege registration is not open right now.").ConfigureAwait(false);