feat(backup): deploy cron-based SQLite backups to production

- Create deploy/xamxam-backup.cron with hourly (30d) and daily (90d) jobs
- Add just recipes for deploying backup infrastructure:
  - deploy-backup-script: upload backup-sqlite.sh to /usr/local/bin
  - deploy-backup-cron: install cron.d file, create /var/backups/xamxam + log
  - deploy-backup: one-shot convenience (script + cron)
  - deploy-check-backup-log: tail the backup log
  - deploy-list-backups: ls remote backup directory
  - trigger-backup: manually invoke backup on server
  - test-restore: scp, gunzip, verify a remote snapshot
- Add reminder to run deploy-backup after first deploy
- Replace 'Contenu (Markdown)' label with 'Syntax Markdown' link (cheatsheet)

scripts/backup-sqlite.sh

@@ -22,7 +22,10 @@ TIMESTAMP=$(date +"%Y-%m-%dT%H-%M-%S")
 BACKUP_FILE="$BACKUP_DIR/db-$TIMESTAMP.db.gz"
 TMP_SNAPSHOT="/tmp/xamxam-snapshot-$$.db"
 
-mkdir -p "$BACKUP_DIR"
+mkdir -p "$BACKUP_DIR" 2>/dev/null || {
+    echo "ERROR: Cannot create backup directory '$BACKUP_DIR'. Run: just deploy-backup-cron" >&2
+    exit 1
+}
 
 # Safe hot backup using SQLite's online backup API
 sqlite3 "$DB_PATH" ".backup $TMP_SNAPSHOT"