feat: auto-create inventory records for new products + migration worker

- CreateNewProductsCommandHandler: inject IInventoryService, auto-init
  inventory record with qty=0 when a new regular product is created
  (DiscountProduct handler already had this)
- InventoryInitializerService: one-time BackgroundService that runs on
  startup, finds existing products without InventoryItem records, and
  creates them (migration for legacy products)
- Register InventoryInitializerService in DI

src/CMSMicroservice.Application/ProductsCQ/Commands/CreateNewProducts/CreateNewProductsCommandHandler.cs

@@ -1,6 +1,7 @@
 using CMSMicroservice.Application.Common.FileManager;
 using CMSMicroservice.Application.Common.Interfaces;
 using CMSMicroservice.Domain.Entities;
+using CMSMicroservice.Domain.Enums;
 using Microsoft.Extensions.Logging;
 
 namespace CMSMicroservice.Application.ProductsCQ.Commands.CreateNewProducts;
@@ -9,15 +10,18 @@ public class CreateNewProductsCommandHandler : IRequestHandler<CreateNewProducts
 {
     private readonly IApplicationDbContext _context;
     private readonly IFileManager _fileManager;
+    private readonly IInventoryService _inventoryService;
     private readonly ILogger<CreateNewProductsCommandHandler> _logger;
 
     public CreateNewProductsCommandHandler(
         IApplicationDbContext context,
         IFileManager fileManager,
+        IInventoryService inventoryService,
         ILogger<CreateNewProductsCommandHandler> logger)
     {
         _context = context;
         _fileManager = fileManager;
+        _inventoryService = inventoryService;
         _logger = logger;
     }
 
@@ -70,6 +74,20 @@ public class CreateNewProductsCommandHandler : IRequestHandler<CreateNewProducts
         await _context.Products.AddAsync(entity, cancellationToken);
         await _context.SaveChangesAsync(cancellationToken);
 
+        // ایجاد رکورد موجودی در سیستم انبارداری
+        try
+        {
+            await _inventoryService.InitializeInventoryAsync(
+                entity.Id,
+                ProductType.RegularProduct,
+                0, // موجودی اولیه صفر — باید از طریق Inventory اضافه شود
+                ct: cancellationToken);
+        }
+        catch (Exception ex)
+        {
+            _logger.LogWarning(ex, "Failed to initialize inventory for Product Id={ProductId}", entity.Id);
+        }
+
         // Handle category assignments
         if (request.CategoryIds is { Count: > 0 })
         {