Calculates the source URL for a delivery receipt at runtime.

This calculation looks up the event module from configuration and calls its source_url/2 callback with a reconstructed context and channel.

The URL is computed at runtime (not persisted) because it may depend on who is viewing it (admin vs user paths differ).

Requirements for source_url to work

For an event to have working source URLs on delivery receipts, the event module must define the data_key/0 callback:

@impl true
def data_key, do: :order  # or :ticket, :user, etc.

The data_key tells the calculation which key to use when reconstructing the context for URL generation. Without it, source_url will return nil.

See AshDispatch.Event documentation for more details.