Migrate canonical source from Gitea to GitHub (#738)

- Update all self-references from `git.vdb.to/cerc-io/stack-orchestrator` to
  `github.com/cerc-io/stack-orchestrator` (setup.py, pyproject.toml, README,
  docs, install scripts, cloud-init scripts, stack READMEs)
- Fix release download URL pattern (`releases/download/latest` -> `releases/latest/download`)
- Port 5 Gitea-only CI workflows to GitHub Actions (k8s-deploy, k8s-deployment-control, container-registry, database, external-stack)
- Pin `shiv==1.0.8` in all workflows for reproducible builds
- Restrict smoke/deploy/webapp test push triggers to `main` only
- Remove `.gitea/` directory - Gitea repo to be archived

Author: prathamesh0