feat: add automated backup system with Docker-based cron

Implement a dedicated Docker container (backup-cron) for automated daily
backups and maintenance tasks, eliminating the need for host cron configuration.

New features:
- backup-cron service: Alpine-based container with Docker CLI and cron
- Automated daily backup at 5:00 AM (Europe/Paris timezone)
- Automated health check at 6:00 AM (after backup)
- Weekly log cleanup on Sundays at 3:00 AM (removes logs >30 days)

Files added:
- cron/Dockerfile: Alpine Linux with docker-cli, bash, and tzdata
- cron/entrypoint.sh: Starts crond and displays configuration
- cron/crontab: Scheduled tasks configuration
- cron/README.md: Complete documentation for automated backups
- scripts/clean-old-logs.sh: Automated log cleanup script

Makefile enhancements:
- make cron-status: Display backup automation status and schedule
- make cron-logs: View logs from automated tasks

Configuration improvements:
- Auto-detect COMPOSE_PROJECT_NAME from directory name (portable)
- Fix df command to use POSIX format (-P flag) for consistent output
- Updated .env.example with COMPOSE_PROJECT_NAME documentation

Benefits:
- No host cron configuration required
- Portable across different environments
- Automatic timezone handling
- Integrated with existing backup/health check scripts
- Logs all automated tasks for monitoring

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

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

docker-compose.yml

@@ -63,6 +63,28 @@ services:
     networks:
       - nextcloud-net
 
+  backup-cron:
+    build: ./cron
+    restart: unless-stopped
+    volumes:
+      # Monter le projet complet pour accéder aux scripts
+      - .:/project
+      # Monter Docker socket pour exécuter les commandes docker-compose
+      - /var/run/docker.sock:/var/run/docker.sock
+      # Partager les volumes avec nextcloud pour les backups
+      - ./data:/var/www/html
+      - ./db:/var/lib/mysql
+      - ./backups:/project/backups
+      - ./logs:/logs
+    environment:
+      - TZ=Europe/Paris
+      - COMPOSE_PROJECT_NAME
+    depends_on:
+      - nextcloud
+      - db
+    networks:
+      - nextcloud-net
+
   db:
     image: mariadb:10.11
     restart: unless-stopped