feat: Gitea Actions CI + env file + Gitea registry image

- .gitea/workflows/docker.yml: builds and pushes image to
  git.albert-zangerl.com/al/logwatch on every push to main
  and on version tags (v*)
- stack.env.example: template for environment variables
- stack.env: gitignored, lives on docker.lan next to the stack
- portainer-stack.yml: updated to use Gitea registry image
  and env_file reference

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -3,4 +3,5 @@ __pycache__/
 *.pyo
 .env
 .env.local
+stack.env
 *.log