feat: support seerr base url

[frzifus]

Pull Request Description

Add support for env variable to define jellyseerr base url.
Using the env variable BASE_URL_SEERR we can pre-fill the jellyseerr server address.

Issue Being Fixed

Resolves #issue-number

Screenshots / Recordings

docker run  --rm --name fladder-test -e BASE_URL=http://localhost:1234 -e BASE_URL_SEERR=http://localhost:5678 -p 8080:8080 localhost/fladder-rootless:test

Checklist

• If a new package was added, did you ensure it works for all supported platforms? Is the package well maintained
• Check that any changes are related to the issue at hand.

[PartyDonut]

Hi thanks for implementing this.

Unfortunately I already implemented it a few days ago just didn't have the chance the properly test and merge it.

So will be closing this one in favor of #817