Add cache-control headers to background images

[RayBB]

Description

We should add cache control headers to the background images. That way each time you load the projects page it doesn't have to fully load those images from scratch.

Probably should be added here: https://github.com/go-vikunja/vikunja/blob/e58d10bc72a35c6bc72ff1fc429b246e8faa2c87/pkg/routes/static.go#L248-L252

This is the url: https://try.vikunja.io/api/v1/projects/4/background

Vikunja Version

Version: v0.23.0

Browser and version

FF 125

Can you reproduce the bug on the Vikunja demo site?

Yes

Screenshots