PostgreSQL-backed LangEx.Store using Ecto.
Assumes the lang_ex_store table has been created via
LangEx.Migration (version 2). Values are encoded with
LangEx.Checkpoint.Serializer, so structs, atoms, and tuples
round-trip exactly.
Config
Graph.compile(builder, store: {LangEx.Store.Postgres, repo: MyApp.Repo})