fix: add missing migration

PluralKit.Core/Database/Migrations/52.sql

@@ -0,0 +1,18 @@
+-- database version 52
+-- messages db updates
+
+create index concurrently messages_by_original on messages(original_mid);
+create index concurrently messages_by_sender on messages(sender);
+
+create table command_messages (
+    mid bigint primary key,
+    channel bigint not null,
+    guild bigint not null,
+    sender bigint not null,
+    original_mid bigint not null
+);
+
+create index concurrently command_messages_by_original on command_messages(original_mid);
+create index concurrently command_messages_by_sender on command_messages(sender);
+
+update info set schema_version = 52;