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.Core/Database/Migrations/29.sql

@@ -0,0 +1,5 @@
+-- schema version 29
+
+alter table systems add column is_deleting bool default false;
+
+update info set schema_version = 29;