Postgres-compatible Ecto storage adapter for Squid Mesh journal runtime state.

Use this adapter when the host app wants the Jido journal runtime persisted in the same Postgres-compatible database boundary as the rest of the application:

config :squid_mesh,
  runtime: :journal,
  read_model: :read_model,
  journal_storage: {SquidMesh.Runtime.Journal.Storage.Ecto, repo: MyApp.Repo}

The adapter implements Jido's checkpoint and append-only thread callbacks. Thread appends are serialized with a row-level lock and honor Jido's :expected_rev optimistic concurrency option.