db: auto-backup before server start + daily cron

- backup-db.sh: copies assets.db to /home/oplabs/backups/canteen-db/
- start.sh: runs backup automatically before every launch
- Daily cron at 3am for unattended backups
- Keeps last 30 backups, prunes oldest

start.sh

@@ -36,6 +36,17 @@ if [ "${CANTEEN_WIPE_DB:-}" = "1" ]; then
   rm -f "$DB_PATH" "${DB_PATH}-shm" "${DB_PATH}-wal"
 fi
 
+# ── Back up existing DB before launch ─────────────────────────────────────────
+if [ -f "$DB_PATH" ]; then
+  BACKUP_DIR="/home/oplabs/backups/canteen-db"
+  mkdir -p "$BACKUP_DIR"
+  TIMESTAMP=$(date +%Y%m%d-%H%M%S)
+  cp "$DB_PATH" "${BACKUP_DIR}/assets.db-${TIMESTAMP}"
+  # Keep last 30 backups
+  ls -t "${BACKUP_DIR}/assets.db-"* 2>/dev/null | tail -n +31 | xargs rm -f 2>/dev/null || true
+  echo "   Backup: ${BACKUP_DIR}/assets.db-${TIMESTAMP}"
+fi
+
 # ── Launch ───────────────────────────────────────────────────────────────────
 echo "🚀 Starting Canteen Asset Tracker on https://0.0.0.0:${PORT}"
 echo "   DB:    $DB_PATH"