fix: add joplin_data volume to joplin stack

Added missing volume definition for Joplin Server data persistence.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

joplin/compose.yaml

@@ -3,6 +3,8 @@ services:
     image: ${JOPLIN_IMAGE:-joplin/server:latest}
     container_name: ${JOPLIN_COMPOSE_PROJECT_NAME}_app
     restart: unless-stopped
+    volumes:
+      - joplin_data:/data
     environment:
       TZ: ${TIMEZONE:-Europe/Berlin}
       APP_PORT: ${JOPLIN_APP_PORT:-22300}
@@ -32,6 +34,10 @@ services:
       - 'traefik.docker.network=${NETWORK_NAME}'
       - 'com.centurylinklabs.watchtower.enable=${WATCHTOWER_LABEL_ENABLE}'
 
+volumes:
+  joplin_data:
+    name: joplin_data
+
 networks:
   compose_network:
     name: ${NETWORK_NAME}