Pos_Invoicing

Description:
Implement core data model and workflows to support multiple organizations, stores, and GST registrations under a tenant. This includes creating organizations, managing multi-state GSTINs, onboarding stores, and linking stores to GSTINs using effective-dated mappings.

The system should handle real-world scenarios like multiple stores sharing a GSTIN and GSTIN changes over time without affecting historical records. It must enforce strict validation rules (organization and state-level), ensure tenant isolation, and maintain audit logs.

This epic also ensures that GSTIN used in documents is locked at the time of posting and remains unchanged for compliance.

Acceptance Criteria:

Tenant can manage multiple organizations and GST registrations
Stores can be created and linked to valid GSTINs
Store–GSTIN mapping supports effective dating
GSTIN changes do not impact historical data
Validation rules enforced (same org, same state, tenant isolation)
Documents store GSTIN at posting and remain immutable
Audit logs available for all GSTIN mapping changes
End-to-end flow works for multi-store and multi-GST scenarios