Email transport via Oban jobs.

Enqueues email delivery jobs for asynchronous sending.

Behavior

1. Checks user preferences (if event is user_configurable)
2. Resolves recipients from context
3. Enqueues Oban jobs for each recipient
4. Updates receipt status to :scheduled

Status Flow

pending → scheduled (job enqueued)
        ↘ skipped (user opted out or no recipients)

Later (in Oban worker):
scheduled → sending (job started)
         → sent (email delivered)
         ↘ failed (delivery error)

Delayed Delivery

Supports delayed delivery via channel's time field:

channel = %Channel{
  transport: :email,
  time: {:in, 300}  # Deliver in 5 minutes
}

Example

receipt = %{
  content: %{
    subject: "Order Created",
    from: "orders@example.com",
    html_body: "<h1>Order #1234</h1>",
    text_body: "Order #1234 created"
  }
}

Email.deliver(receipt, context, channel, event_config)
# -> Enqueues Oban job
# -> Returns {:ok, updated_receipt}