docs: add Portainer stack YAML

For deployment on docker.lan via Portainer Web Editor.
Requires image built locally: docker build -t logwatch:latest .

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

portainer-stack.yml

@@ -0,0 +1,11 @@
+services:
+  logwatch:
+    image: 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