Pos_Invoicing

Description:
Implement backend support for Organization and GST Registration under a tenant. This includes creating organizations, adding multiple GSTINs per organization, validating GSTIN format, and ensuring proper mapping with tenant and state.

This acts as the base layer for store creation and GSTIN mapping.

Acceptance Criteria:

Organization must have legal_name (mandatory)
One tenant can have multiple organizations
GSTRegistration must belong to an organization and tenant
GSTIN must be 15 characters and validated
GSTIN must be unique within a tenant
GSTIN state auto-derived from GSTIN
State override allowed with warning flag
Tenant isolation enforced in all queries
Organization and GSTRegistration CRUD APIs available