Pos_Invoicing

Description:
Implement product master, purchase workflow, inventory updates, and input GST (ITC) tracking. This includes product catalog creation, vendor management, purchase invoice processing with line-level GST, goods receipt (GRN) for stock updates, purchase returns with credit notes, and ITC ledger for GST input tracking.

The system must ensure stock updates only on actual receipt (GRN), maintain document immutability, and support accurate GST input calculation for reporting and compliance.

Acceptance Criteria:

Products can be created with HSN, GST rate, and units
Purchase invoices support line-level GST calculation
Stock increases only after GRN posting
Purchase returns reduce stock correctly
Credit notes adjust GST input values
ITC ledger tracks available and reversed input tax
All documents are immutable after posting
Data integrates correctly with GST reports