Add scheduled backups of database and uploads

The backend now writes a timestamped tar.gz of the SQLite snapshot and all
per-order files to BACKUP_DIR on a cron schedule (default 03:00), keeps the
newest BACKUP_KEEP archives, and catches up once on start if a slot was
missed. Configurable via BACKUP_CRON / BACKUP_KEEP / BACKUP_DIR / TZ.
Includes user-facing restore docs in docs/backups.md.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

README.md

@@ -27,11 +27,13 @@ services:
     image: git.sgruber.at/lunchtime/lunchtime-web:backend-latest
     volumes:
       - ./.data:/app/data
+      - ./.backups:/app/backups
       - ./config.yaml:/app/config.yaml:ro
     environment:
       - APP_ENV=production
       - LOG_LEVEL=info
       - DB_PATH=/app/data
+      - TZ=Europe/Vienna # backup schedule follows this timezone
     restart: unless-stopped
 
   frontend:
@@ -47,6 +49,10 @@ docker compose up -d
 
 Open `http://localhost:8020`.
 
+## Backups
+
+The backend writes a nightly archive of the database and all uploaded files to `/app/backups` (mount it, as above). Schedule, retention and restore steps: [docs/backups.md](docs/backups.md).
+
 ## Development
 
 ### Quick Start with Docker Compose