[Feature Request] IMAP Consumer should (must) also mark emails whose attachments are already known

Description Currently, the IMAP Consumer only sets the "processed" flag (IMAP Flag) if the complete import of an email was successful. This is problematic if the attachment already exists in Paperless (duplicate) or is not imported for other reasons. Current behavior: Paperless finds an unmarked email. It is downloaded completely (traffic). If the attachment is already marked as "known," it is not imported. Because the import thus "failed," the email is not marked. In the next scan, the email is downloaded again—essentially an endless loop. Impact: Emails with already known content are unnecessarily downloaded again with every email scan. In our case, this means constant traffic per scan, even if no new emails are added. This makes it impossible to use with larger, existing archives. Expectation: The flag should be set as soon as the email has been successfully read – regardless of whether its attachments were ultimately imported or not. T…