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/CastleSiegeConfiguration.cs

@@ -87,6 +87,13 @@ public class CastleSiegeConfiguration
     /// </summary>
     public int RegistrationFee { get; set; } = 100000;
 
+    /// <summary>
+    /// Gets or sets the game server (SW) id on which the Castle Siege runs. With multiple game servers each
+    /// has its own map instances, so the siege must happen on ONE designated server; the others skip it (they
+    /// only honor the shared castle owner for the rewards). Players must be on this server to take part.
+    /// </summary>
+    public byte CastleSiegeServerId { get; set; }
+
     // --- Runtime fields (hidden from the AdminPanel; proxied by the friendly properties above) ---
 
     /// <summary>Gets or sets the days of week registration auto-opens (UTC). Empty = every day.</summary>

src/GameLogic/CastleSiege/CastleSiegeContext.cs

@@ -158,6 +158,16 @@ public class CastleSiegeContext
         this._dirty = true;
     }
 
+    /// <summary>
+    /// Mirrors the shared castle owner from the configuration. Used on game servers that do NOT host the
+    /// siege, so their hunting-map gate and castle flag still reflect the current owner. Does not mark the
+    /// state dirty (these servers never persist).
+    /// </summary>
+    public void SyncOwnerFromConfig()
+    {
+        this.OwnerGuildName = this.Configuration.PersistedOwnerGuildName;
+    }
+
     /// <summary>
     /// Sets the weekly auto-schedule (days of week + UTC time) into the configuration and marks the state
     /// dirty for persistence. Empty days disables auto-start (manual only). The configuration is the single

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);