Optional-deps gateway for Oban (D-02, D-37 canonical pattern). Lets the Phase 21 worker (Relyra.Workers.MetadataRefresh) and the documented Oban Cron one-liner reference Oban modules even when Oban is not in the adopter's deps tree.

The @compile {:no_warn_undefined, [...]} attribute keeps the mix compile --no-optional-deps --warnings-as-errors CI lane green (engineering-DNA §3 invariant).

Phase 21 deviates from Relyra.LiveAdmin's raise ArgumentError shape because the Phase-21 callers (the worker perform/1 callback) expect the {:ok | :error, %Relyra.Error{}} result-tuple discipline. Adopters who want a hard crash can pattern-match {:error, _} and re-raise.