- warehouses.service.ts: Add code uniqueness check with tenantId
- products.service.ts: Add SKU/barcode uniqueness checks with tenantId
- accounts.service.ts: Add tenantId to code and parent validation
Fixes 5 RLS gaps in backend services for multi-tenant isolation.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>