feat: alternate proxy switch mode "add"

PluralKit.Core/Database/Migrations/49.sql

@@ -0,0 +1,11 @@
+-- database version 49
+-- change proxy switch config to an enum
+
+alter table system_config 
+    alter column proxy_switch drop default,
+    alter column proxy_switch type int
+        using case when proxy_switch then 1 else 0 end,
+    alter column proxy_switch set default 0,
+    add constraint proxy_switch_check check (proxy_switch = ANY (ARRAY[0,1,2]));
+
+update info set schema_version = 49;