Convert indexer from simple task async to oban job

[chadfennell]

We have code like this, which is fine at a small scale:

    |> Repo.all()
    |> Task.async_stream(&index_decision(&1, opts), max_concurrency: 10, timeout: :infinity)
    |> Stream.run()

We'll want to index in batches to handle full reindex of large collections.