Support for importing S/MIME‑signed emails

Description Summary Paperless‑NGX currently cannot import emails that are S/MIME‑signed (e.g., multipart/signed or application/pkcs7-signature). These messages are skipped or rejected by the mail consumer, even though the email content itself is not encrypted and remains fully readable. Problem Many organizations use S/MIME signatures by default to ensure message integrity. While the message body and attachments are not encrypted, Paperless‑NGX treats signed emails as unsupported and does not process them. This prevents automated ingestion of documents that arrive via signed email channels. Expected behavior Paperless‑NGX should: • accept and import signed but unencrypted S/MIME emails, • extract the message body and attachments normally, • ignore the signature part unless validation is explicitly implemented later, • optionally store a metadata flag such as signed: true. Signature validation is not required for this feature request; the goal i…