Support for local storage

Problem Statement

It would be nice if kaneo could support local storage / docker volume.

Proposed Solution

Not sure how the storage is currently managed but I can imagine an environment variable like LOCAL_STORAGE_ENABLE=true assisted by configuring a docker volue in compose file.

Alternative Solutions

N/A

Relevant Context

Spinning whole S3 object storage in a small homelab is just too big hassle, especially that I expect to use attachments rarely.

Does this feature align with Kaneo's focus on simplicity?

It definitely adds additional code for maintenance but the philosophy and simplicity of the solution won't be affected.

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Originally requested by @skibbipl on 2026-08-21. Original GitHub request #1637.