Merge pull request 'feat: enhance stage quantity management for running stock inputs' (#48) from bug-fix-production into Dev

Reviewed-on: #48

Backend/ERPCore/Dtos/Production/RunDtos.cs

@@ -99,8 +99,10 @@ public sealed class CreateRunRequest
 }
 
 /// <summary>
-/// Per-run scaling override (FR-MFG-08). Only accepted while the stage has not started
-/// (<c>409 STAGE_NOT_EDITABLE</c>).
+/// Per-run scaling override (FR-MFG-08). Accepted before the stage starts, and — for a Stock
+/// input only — while it is running too: raising <see cref="StageQuantityLine.PlannedQty"/>
+/// draws the extra amount from FIFO, lowering it returns the excess (<c>409
+/// STAGE_NOT_EDITABLE</c> for every other status).
 /// </summary>
 public sealed class UpdateStageQuantitiesRequest
 {
@@ -116,4 +118,11 @@ public sealed class StageQuantityLine
 
     [Range(0.0001, double.MaxValue)]
     public decimal PlannedQty { get; set; }
+
+    /// <summary>
+    /// Required only when this line lowers a running Stock input below what it has already
+    /// consumed — the excess is returned to stock the same way <c>ReturnLeftoverAsync</c>
+    /// does, and that always needs a reason (FR-MFG-14). Ignored otherwise.
+    /// </summary>
+    public int? ReasonCodeId { get; set; }
 }