feat: add jellyfin image source

[UtkarshVerma]

Description

This PR adds Jellyfin as an image source, the same way as it had been done for Navidrome.

Related Issue(s)

#327

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

• Unit Tests
• Manual Testing (please describe steps)

I hooked up Koito to Jellyfin and it was able to get the images.

Checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have ensured my changes generate no new warnings

AI/LLM Usage

• I disclose that 100% of the code in this PR is written by an LLM. (please estimate)
• I fully understand all changes made by LLM-generated code.
• I have not and will not use an LLM to write any part of this PR description or PR comments.

Regarding the AI usage, I thoroughly reviewed the code so it follows your architecture and I have experience with Go applications. I used the LLM since these kinds of jobs (API implementations) are perfect for it.

Screenshots (if applicable)