Default Accrue.Mailer adapter — enqueues an Oban job on the :accrue_mailers queue that a worker later turns into a delivered email.

Pay-style override ladder

This adapter implements a Pay-inspired four-rung override ladder:

1. Kill switch (:emails config) — Accrue.Mailer.deliver/2 short-circuits before this adapter is reached. Handled at the behaviour layer.
2. MFA conditional (:emails value is {Mod, :fun, args}).
3. Template module override (:email_overrides config) — resolved in Accrue.Workers.Mailer.resolve_template/1.
4. Full pipeline replace (:mailer config pointing at a custom module) — already supported via Accrue.Mailer.impl/0.

Oban args safety

Oban persists job args as JSONB. The assigns map MUST contain only scalars (no structs, pids, functions, refs) so a worker crash mid-job doesn't corrupt the queue and so sensitive structs don't leak into the oban_jobs table. only_scalars!/1 walks the map and raises ArgumentError on any non-primitive value. The convention is: pass entity IDs, not entity structs. The worker rehydrates at delivery time.