Database schema changes included as Laravel migrations only. No test data is included.

1) 2026_07_08_101500_create_sync_v2_tables_and_extend_sales_invoices.php
   - Creates: sync_requests, sync_request_items, sync_devices, sync_heartbeats
   - Adds guarded Sync V2 columns and indexes to sales_invoices

2) 2026_07_08_103000_create_sales_returns_table.php
   - Creates sales_returns when absent

3) 2026_07_10_180500_create_supplier_payments_table.php
   - Creates supplier_payments when absent
   - Structure matches SHOW CREATE TABLE supplied from the test database

All three migrations use table/column existence guards. They do not copy, truncate,
delete, or replace production data.