add db/schema.sql

Not super necessary, but there's a [bug in pliny](https://github.com/interagent/pliny/pull/73) causing issues to run `bin/setup` on apps without a schema. This would also make that setup a bit faster.
Ah! This should not be making to the schema per https://github.com/interagent/pliny/pull/50/files
I'll take a look.