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

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

PluralKit.Core/Database/Migrations/32.sql

@@ -0,0 +1,6 @@
+-- database version 32
+-- re-add last message timestamp to members
+
+alter table members add column last_message_timestamp timestamp;
+
+update info set schema_version = 32;