Pos_Invoicing

Description:
Create Organization entity under a tenant and allow multiple GST registrations per organization. GSTIN should be validated and mapped to the correct state. Ensure tenant-level isolation and uniqueness of GSTIN.

Acceptance Criteria:

Organization must have legal_name
One tenant can have multiple organizations
GSTIN must be unique within a tenant
GSTRegistration must belong to same tenant & organization
GSTIN state auto-derived (override allowed with warning)
Tenant isolation enforced in queries