Feature request: embed invoice documents as attachments in the e-invoice XML (BG-24 / AdditionalReferencedDocument)

Feature request: embed invoice documents as attachments in the e-invoice XML (BG-24 / AdditionalReferencedDocument) Invoice Ninja version: v5.13.x (self-hosted) E-invoice format: Factur-X / ZUGFeRD (CII) Problem German corporate invoice intake systems increasingly require that supporting documents are embedded inside the e-invoice itself, not sent as separate files. Example requirements received from recipients (enviaM group, Kaufland, E.ON): • Only one file per email — either an XML or a ZUGFeRD PDF. • No additional files in the email. • Any supporting documents must be embedded in the original invoice. EN 16931 provides exactly this: BG-24 "Additional supporting documents", in CII as ram:AdditionalReferencedDocument with ram:AttachmentBinaryObject (BT-125). Invoice Ninja currently offers no way to produce this. Documents attached to an invoice are either: • merged into the PDF as extra pages (UI upload), or • sent as s…