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>

docker-compose.yml

@@ -6,9 +6,9 @@ services:
     restart: unless-stopped
     environment:
       # ntfy source channel (your Watchtower notifications)
-      - SOURCE_URL=http://192.168.123.77/albert
+      - SOURCE_URL=https://ntfy.albert-zangerl.com/albert
       # ntfy target channel (where important stuff goes)
-      - TARGET_URL=http://192.168.123.77/wichtig
+      - TARGET_URL=https://ntfy.albert-zangerl.com/wichtig
       # optional: Bearer token if target channel needs auth
       # - TARGET_TOKEN=your-token-here
       - LOG_LEVEL=INFO