refactor: use env var substitution for Portainer Repository stack

Variables now use \${VAR:-default} syntax so they can be set
via Portainer's environment editor without modifying the compose file.

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

docker-compose.yml

@@ -5,11 +5,8 @@ services:
     container_name: logwatch
     restart: unless-stopped
     environment:
-      # ntfy source channel (your Watchtower notifications)
-      - SOURCE_URL=https://ntfy.albert-zangerl.com/albert
-      # ntfy target channel (where important stuff goes)
-      - TARGET_URL=https://ntfy.albert-zangerl.com/wichtig
-      # optional: Bearer token if target channel needs auth
-      # - TARGET_TOKEN=your-token-here
-      - LOG_LEVEL=INFO
-      - RECONNECT_DELAY=10
+      - SOURCE_URL=${SOURCE_URL:-https://ntfy.albert-zangerl.com/albert}
+      - TARGET_URL=${TARGET_URL:-https://ntfy.albert-zangerl.com/wichtig}
+      - TARGET_TOKEN=${TARGET_TOKEN:-}
+      - LOG_LEVEL=${LOG_LEVEL:-INFO}
+      - RECONNECT_DELAY=${RECONNECT_DELAY:-10}