Support for importing S/MIME‑encrypted emails

Description Summary Paperless‑NGX currently cannot process S/MIME‑encrypted emails. When an incoming message uses S/MIME encryption (application/pkcs7-mime with smime-type=enveloped-data), the mail consumer cannot extract the message body or attachments. As a result, encrypted emails cannot be used as a document source, even when the user has access to the necessary private key. Problem S/MIME encryption is widely used in corporate, governmental, and regulated environments to ensure confidentiality. Many organizations send invoices, statements, and other documents exclusively via encrypted email. Paperless‑NGX cannot ingest these messages, which breaks automated workflows and forces users to manually decrypt and forward emails before import. Expected behavior Paperless‑NGX should be able to: • accept S/MIME‑encrypted emails, • decrypt them using a user‑provided private key and certificate, • extract the message body and attachments after decryption, &…