storage: add initial postgres event persistence and schema migration

lib/parrhesia/repo.ex

@@ -0,0 +1,12 @@
+defmodule Parrhesia.Repo do
+  @moduledoc """
+  PostgreSQL repository for storage adapter persistence.
+
+  Note: the repo is not yet started by the supervision tree while the
+  storage adapter is in staged implementation.
+  """
+
+  use Ecto.Repo,
+    otp_app: :parrhesia,
+    adapter: Ecto.Adapters.Postgres
+end