[Feature]: Application-level encryption at rest

[Eucliwood090]

Is your feature request related to a problem?

Currently, Mail-Archiver relies entirely on the underlying infrastructure (disk/filesystem encryption) to protect archived emails at rest. Unlike tools such as Nextcloud, there's no application-level encryption with key management.

Describe the solution you'd like

Given that archived emails are often sensitive (legal, financial, personal data), it would be valuable to have an option for encrypting message content/attachments in the database using a user-managed key — even a basic implementation (e.g., encrypting the body/attachment blobs with a key derived from a passphrase) would meaningfully improve the security posture for self-hosted deployments where infra-level encryption isn't always guaranteed.

Additional context

No response