ci: migrate from ghcr.io to Gitea built-in registry (#25)
CAR-995: Update CI workflow to use Gitea built-in container registry.
- REGISTRY env var: ghcr.io -> git.farh.net
- Replace Docker Hub/GHCR login with direct docker login using github.token
- Remove Docker Hub credentials from service containers
- Update deploy kustomize image refs to use env vars