fix: set local adapter SERVER_URL default (supersedes #55)

[xMinhx]

Summary

• Set SERVER_URL in backend/env.template to http://host.docker.internal:8000 for local adapter usage.
• Add ADDITIONAL_ALLOWED_HOSTS=host.docker.internal to avoid host validation issues in local Docker-based calls.

Replacement rationale

• Supersedes fixed warning about missing env variable by setting default value #55.
• The original PR branch is based on an older workflow/branch state.
• This replacement PR is rebased on current main so CI checks can run on current infrastructure and merge risk is reduced.
• No new features

Notes

• Original contribution preserved via cherry-pick from the source commit.

[xMinhx]

This seems like a good follow-up to #60 . It keeps the replacement for #55 moving once the working CI workflow is back.