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/Controllers/PurchaseOrdersController.cs

@@ -57,6 +57,25 @@ public sealed class PurchaseOrdersController : ApiControllerBase
         return Ok(result.Value);
     }
 
+    /// <summary>Submit a Draft PO (Draft → Approved). 409 PO_NOT_EDITABLE if not Draft.</summary>
+    [HttpPost("{poId:int}/submit")]
+    [ProducesResponseType(typeof(PurchaseOrderDto), StatusCodes.Status200OK)]
+    [ProducesResponseType(StatusCodes.Status404NotFound)]
+    [ProducesResponseType(StatusCodes.Status409Conflict)]
+    public async Task<ActionResult<PurchaseOrderDto>> Submit(int poId, CancellationToken ct)
+        => Ok(await _pos.SubmitAsync(poId, ct));
+
+    /// <summary>Delete a PO — permitted only while Draft (409 PO_NOT_EDITABLE otherwise).</summary>
+    [HttpDelete("{poId:int}")]
+    [ProducesResponseType(StatusCodes.Status204NoContent)]
+    [ProducesResponseType(StatusCodes.Status404NotFound)]
+    [ProducesResponseType(StatusCodes.Status409Conflict)]
+    public async Task<IActionResult> Delete(int poId, CancellationToken ct)
+    {
+        await _pos.DeleteAsync(poId, ct);
+        return NoContent();
+    }
+
     /// <summary>Approve — no-op in Phase 1 (POs auto-approve); transitions PendingApproval→Approved when enabled.</summary>
     [HttpPost("{poId:int}/approve")]
     [ProducesResponseType(typeof(PurchaseOrderDto), StatusCodes.Status200OK)]