feat(procurement): enhance purchase order and GRN functionalities

- Updated purchase order descriptions for clarity on draft and submission processes.
- Implemented submit and delete functionalities for draft purchase orders, allowing users to manage their orders more effectively.
- Added discount and VAT fields to GRN lines, enabling better cost tracking and reporting.
- Enhanced validation for GRN lines to ensure discount and VAT percentages are within acceptable ranges.
- Updated API to support new functionalities, including submitting and deleting purchase orders.
- Improved UI components for better user experience in managing purchase orders and GRNs.
- Documented changes in security and backend phase documentation to reflect new processes and requirements.

Backend/ERPCore/Domain/Entities/GrnLine.cs

@@ -3,9 +3,13 @@ using ERPCore.Domain.Enums;
 namespace ERPCore.Domain.Entities;
 
 /// <summary>
-/// GRN line (FR-GRN-04..08). <see cref="UnitCost"/> is the PO-derived cost for
-/// PO-based receipts (client cost ignored — 02-SECURITY C.3) or the entered cost
-/// for direct receipts. <see cref="ReceivedValue"/> = qty × unitCost.
+/// GRN line (FR-GRN-04..08). <see cref="UnitCost"/> is the gross cost received at:
+/// entered on the line, defaulting to the PO price when omitted (a per-receipt price
+/// override is now permitted — see docs/02-SECURITY C.3, revised). <see cref="PoUnitPrice"/>
+/// snapshots the PO price at receipt so the variance survives later PO edits.
+/// <see cref="NetUnitCost"/> = unitCost after trade discount — this is what the FIFO layer
+/// costs at (VAT never enters stock value; it is recoverable input tax).
+/// <see cref="ReceivedValue"/> = qty × netUnitCost (after discount, before VAT).
 /// <see cref="HoldStatus"/> gates issuability. Model: docs/10 Part C.3.
 /// </summary>
 public class GrnLine
@@ -31,7 +35,30 @@ public class GrnLine
     public Batch? Batch { get; set; }
 
     public decimal Qty { get; set; }
+
+    /// <summary>Gross unit cost received at (entered, or PO price when omitted).</summary>
     public decimal UnitCost { get; set; }
+
+    /// <summary>Snapshot of the PO line price at receipt; null for direct receipts.</summary>
+    public decimal? PoUnitPrice { get; set; }
+
+    /// <summary>Trade discount percentage (0–100), entered.</summary>
+    public decimal DiscountPct { get; set; }
+
+    /// <summary>UnitCost × (1 − DiscountPct/100) — the inventory (FIFO layer) cost.</summary>
+    public decimal NetUnitCost { get; set; }
+
+    /// <summary>VAT percentage (0–100), entered. Recoverable — does not affect stock value.</summary>
+    public decimal VatPct { get; set; }
+
+    /// <summary>Qty × NetUnitCost × VatPct/100.</summary>
+    public decimal VatAmount { get; set; }
+
+    /// <summary>Qty × NetUnitCost (after discount, before VAT).</summary>
     public decimal ReceivedValue { get; set; }
+
+    /// <summary>Qty × NetUnitCost + VatAmount — payable to the vendor.</summary>
+    public decimal LineTotal { get; set; }
+
     public HoldStatus HoldStatus { get; set; } = HoldStatus.Available;
 }

Backend/ERPCore/Domain/Entities/PoLine.cs

@@ -22,7 +22,7 @@ public class PoLine
     public Warehouse? Warehouse { get; set; }
 
     public decimal Qty { get; set; }
-    public decimal UnitPrice { get; set; }
+    public decimal UnitPrice { get; set; }//
     public decimal Tax { get; set; }
     public decimal QtyReceived { get; set; }
 }