Null Accrue.PDF adapter for Chrome-hostile deploys (D6-06).
Returns {:error, %Accrue.Error.PdfDisabled{}} without raising.
Invoice email workers pattern-match on this tagged error and fall
through to including the Stripe hosted_invoice_url as a link
instead of attaching a rendered PDF binary — graceful degradation
satisfies Phase 6 SC #4.
Log level is fixed at :debug (locked by D6-06) — this is an
expected, configuration-driven branch, not an error or warning
condition.