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>

portainer-stack.yml

@@ -1,11 +1,7 @@
 services:
   logwatch:
-    image: logwatch:latest
+    image: git.albert-zangerl.com/al/logwatch:latest
     container_name: logwatch
     restart: unless-stopped
-    environment:
-      - SOURCE_URL=https://ntfy.albert-zangerl.com/albert
-      - TARGET_URL=https://ntfy.albert-zangerl.com/wichtig
-      # - TARGET_TOKEN=your-token-here
-      - LOG_LEVEL=INFO
-      - RECONNECT_DELAY=10
+    env_file:
+      - stack.env