feat: block running commands / proxying when system is being deleted

Large systems take way too long to delete, which causes any insert for that system to block a connection, which exhausts the connection pool

PluralKit.Bot/Handlers/MessageCreated.cs

@@ -120,6 +120,17 @@ public class MessageCreated: IEventHandler<MessageCreateEvent>
         if (!HasCommandPrefix(content, ourUserId, out var cmdStart) || cmdStart == content.Length)
             return false;
 
+        if (ctx.IsDeleting)
+        {
+            await _rest.CreateMessage(evt.ChannelId, new()
+            {
+                Content = $"{Emojis.Error} Your system is currently being deleted."
+                    + " Due to database issues, it is not possible to use commands while a system is being deleted. Please wait a few minutes and try again.",
+                MessageReference = new(guild?.Id, channel.Id, evt.Id)
+            });
+            return true;
+        }
+
         // Trim leading whitespace from command without actually modifying the string
         // This just moves the argPos pointer by however much whitespace is at the start of the post-argPos string
         var trimStartLengthDiff =
@@ -164,6 +175,8 @@ public class MessageCreated: IEventHandler<MessageCreateEvent>
     private async ValueTask<bool> TryHandleProxy(MessageCreateEvent evt, Guild guild, Channel channel,
                                                  MessageContext ctx)
     {
+        if (ctx.IsDeleting) return false;
+
         var botPermissions = await _cache.PermissionsIn(channel.Id);
 
         try