feature: add ffmpeg

[realies]

It would be nice if I don't have to carry my own CI for this.

I understand the security considerations around preview generation mentioned in the examples documentation.
However, I believe FFmpeg belongs in the base image alongside ImageMagick because:

1. It's already a recommended component in the official "full" example
2. Like ImageMagick, it's a core media processing dependency
3. Users can still disable preview generation for high-security deployments
4. Having it in the base image reduces duplicate CI efforts while maintaining the ability to control its usage

The change is minimal and follows the pattern of including essential media processing tools while letting administrators control their usage through Nextcloud's configuration.

[joshtrichards]

It also adds something like ~500MB to the image size last I checked. ;-)

Between that and:

• it being easy enough to do a build directly within a Compose file (no need to build+upload an image via CI)
• more regular use of Imaginary for offloading preview generation these days with Nextcloud (which embraces the micro-services model)

I'm not sure if it's even as important as it once was. So I'm currently against adding it to the base image.

[realies]

It adds 133.56 MB to the amd64 and 120.19 MB to the arm64 latest images right now.

• https://hub.docker.com/_/nextcloud/tags?name=latest
• https://hub.docker.com/r/realies/nextcloud/tags?name=latest

It's not particularly difficult to make custom solutions for this, but:

• doing a build directly within a Compose file does not work great with a lot of auto-update systems that just check upstream for a new image
• does not depend on other services
• is not a workaround

The idea of this PR is to bring a dependency for a Nextcloud functionality that requires it.
Similarly to how ImageMagick is included.