schema: change premium to premium_since

sql for instances:

```sql
-- remove old nitro column
ALTER TABLE users DROP COLUMN premium;

-- add new nitro column
ALTER TABLE users ADD COLUMN premium_since timestamp without time zone DEFAULT NULL;
```

schema.sql

@@ -69,7 +69,7 @@ CREATE TABLE IF NOT EXISTS users (
     flags int DEFAULT 0,
 
     -- nitro status encoded in here
-    premium bool DEFAULT false,
+    premium_since timestamp without time zone default NULL,
 
     -- private info
     phone varchar(60) DEFAULT '',
@@ -78,6 +78,7 @@ CREATE TABLE IF NOT EXISTS users (
     PRIMARY KEY (id, username, discriminator)
 );
 
+
 -- main user settings
 CREATE TABLE IF NOT EXISTS user_settings (
     id bigint REFERENCES users (id),