Implement automated store/whatsapp.db backup

[FelixIsaac]

Periodic backup of store/whatsapp.db (session keys/encryption material) to prevent forced relink on corruption.

Why: DB corruption = forced device relink, accidental deletion protection, disk failure recovery.

Implementation (simple):

0 */6 * * * cp /app/store/whatsapp.db /app/store/whatsapp.db.backup-$(date +%Y%m%d-%H%M)
0 0 * * * find /app/store -name "whatsapp.db.backup-*" -mtime +7 -delete

Effort: ~15 minutes (simple), ~2 hours (production with S3/encryption)
Priority: High value, low effort