Library-owned schema migration (Oban-style).

The host application does not copy the DDL; it writes a one-line Ecto migration that delegates here:

defmodule MyApp.Repo.Migrations.SetupGenDurable do
  use Ecto.Migration

  def up,   do: GenDurable.Migration.up()
  def down, do: GenDurable.Migration.down()
end

Options

• :prefix — Postgres schema the tables live in (default "public").
• :version — schema version to migrate to (default: latest for up/1, 0 for down/1).

The installed schema version is recorded in COMMENT ON TABLE gen_durable, so up/1 only applies the increments that are missing. This keeps the host-facing call stable as the schema evolves across library releases.