fix(bot): add callback to update message count after proxying

also re-add -with-last-message flag :D

PluralKit.Core/Database/Views/ListedMember.cs

@@ -6,7 +6,6 @@ namespace PluralKit.Core;
 // TODO: is inheritance here correct?
 public class ListedMember: PKMember
 {
-    // public ulong? LastMessage { get; }
     public Instant? LastSwitchTime { get; }
 
     public AnnualDate? AnnualBirthday =>

PluralKit.Core/Database/Views/views.sql

@@ -9,10 +9,6 @@ from systems
 
 create view member_list as
 select members.*,
-       -- Find last message ID
-       -- max(mid) does full table scan, order by/limit uses index (dunno why, but it works!)
-       --    (select mid from messages where messages.member = members.id order by mid desc nulls last limit 1) as last_message,
-
        -- Find last switch timestamp
        (
            select max(switches.timestamp)