ci: migrate from GitHub Actions to Gitea Actions

Move workflows to .gitea/workflows and adapt for git.farh.net:
- Push container images to git.farh.net instead of GHCR/Docker Hub
- Publish Helm chart as OCI artifact (no gh-pages, Gitea lacks Pages)
- Replace cosign keyless signing with key-based (COSIGN_PRIVATE_KEY/PASSWORD/PUBLIC_KEY)
- Swap @semantic-release/github for semantic-release-gitea
- Drop gh CLI from rollback workflow
- Use GITEA_TOKEN for registry auth and release creation
- Add Artifact Hub annotations to Chart.yaml
- Run on ubuntu-latest

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -5,3 +5,5 @@ credentials/
 dist/
 repos/
 .turbo/
+cosign.key
+cosign.pub