Add Stock Tracking endpoint to Inventory

Adds GET /client/inventory/stock-movements, a unified read-only ledger
flattening every GRN line, sale line and return across all products
into one shape for the new Stock Tracking tab's table.

DTOs/Inventory/InventoryDtos.cs

@@ -85,6 +85,27 @@ namespace Fitness_core_BE.DTOs.Inventory
         public List<SaleItemDto> Items { get; set; } = [];
     }
 
+    /// <summary>One row in the Stock Tracking tab's unified ledger -- every GRN line, sale line
+    /// and return across every product, flattened into one shape for a single filterable table.
+    /// Not persisted; built on read from GRNs/Sales/Returns.</summary>
+    public class StockMovementDto
+    {
+        /// <summary>"GRN" | "Sale" | "Return".</summary>
+        public string Type { get; set; } = string.Empty;
+        public Guid ProductId { get; set; }
+        public string ProductName { get; set; } = string.Empty;
+        /// <summary>The GRN/Sale number this line belongs to, or the return's Category label.</summary>
+        public string Reference { get; set; } = string.Empty;
+        /// <summary>Only set for Type == "Return" -- the write-off/return category.</summary>
+        public string? Category { get; set; }
+        /// <summary>Signed: positive for a GRN or a CustomerReturn (stock in), negative for a
+        /// Sale or any other return category (stock out).</summary>
+        public decimal QuantityChange { get; set; }
+        /// <summary>This line's monetary value -- a GRN/Sale line's LineTotal, or a return's Amount.</summary>
+        public decimal Amount { get; set; }
+        public DateTime Date { get; set; }
+    }
+
     public class StockReturnDto
     {
         public Guid StockReturnId { get; set; }