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.Infrastructure/BackgroundServices/InventoryInitializerService.cs

@@ -0,0 +1,153 @@
+using CMSMicroservice.Application.Common.Interfaces;
+using CMSMicroservice.Domain.Enums;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Hosting;
+using Microsoft.Extensions.Logging;
+
+namespace CMSMicroservice.Infrastructure.BackgroundServices;
+
+/// <summary>
+/// سرویس یکباره (Migration) برای مقداردهی اولیه انبارداری از محصولات قدیمی.
+/// محصولاتی که قبل از فعال‌سازی سیستم انبارداری ایجاد شده‌اند رکورد InventoryItem ندارند.
+/// این Worker در استارتاپ اجرا شده، برای آنها رکورد می‌سازد و سپس متوقف می‌شود.
+/// توجه: محصولات جدید از طریق CreateProductCommandHandler خودکار رکورد انبار دریافت می‌کنند.
+/// این سرویس را می‌توانید بعد از اجرای اولیه از DI حذف کنید.
+/// </summary>
+public class InventoryInitializerService : BackgroundService
+{
+    private readonly IServiceScopeFactory _scopeFactory;
+    private readonly ILogger<InventoryInitializerService> _logger;
+
+    public InventoryInitializerService(
+        IServiceScopeFactory scopeFactory,
+        ILogger<InventoryInitializerService> logger)
+    {
+        _scopeFactory = scopeFactory;
+        _logger = logger;
+    }
+
+    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
+    {
+        // صبر کوتاه برای اطمینان از آماده شدن دیتابیس
+        await Task.Delay(TimeSpan.FromSeconds(10), stoppingToken);
+
+        _logger.LogInformation("InventoryInitializerService started — scanning products for missing inventory records...");
+
+        try
+        {
+            using var scope = _scopeFactory.CreateScope();
+            var context = scope.ServiceProvider.GetRequiredService<IApplicationDbContext>();
+            var inventoryService = scope.ServiceProvider.GetRequiredService<IInventoryService>();
+
+            var (regularCount, discountCount) = await InitializeAllProducts(context, inventoryService, stoppingToken);
+
+            _logger.LogInformation(
+                "InventoryInitializerService completed — initialized {RegularCount} regular products and {DiscountCount} discount products",
+                regularCount, discountCount);
+        }
+        catch (Exception ex)
+        {
+            _logger.LogError(ex, "InventoryInitializerService encountered an error");
+        }
+
+        // این Worker فقط یکبار اجرا می‌شود
+        _logger.LogInformation("InventoryInitializerService finished — shutting down (one-time execution)");
+    }
+
+    private async Task<(int regularCount, int discountCount)> InitializeAllProducts(
+        IApplicationDbContext context,
+        IInventoryService inventoryService,
+        CancellationToken ct)
+    {
+        var regularCount = 0;
+        var discountCount = 0;
+
+        // ── محصولات عادی ──
+        // محصولاتی که هنوز InventoryItem برایشان ایجاد نشده
+        var existingRegularProductIds = await context.InventoryItems
+            .Where(i => !i.IsDeleted && i.ProductId != null)
+            .Select(i => i.ProductId!.Value)
+            .ToListAsync(ct);
+
+        var regularProducts = await context.Products
+            .Where(p => !p.IsDeleted && !existingRegularProductIds.Contains(p.Id))
+            .Select(p => new { p.Id, p.Title, p.RemainingCount })
+            .ToListAsync(ct);
+
+        _logger.LogInformation("Found {Count} regular products without inventory records", regularProducts.Count);
+
+        foreach (var product in regularProducts)
+        {
+            if (ct.IsCancellationRequested) break;
+
+            try
+            {
+                var initialQty = Math.Max(0, product.RemainingCount);
+
+                await inventoryService.InitializeInventoryAsync(
+                    product.Id,
+                    ProductType.RegularProduct,
+                    initialQty,
+                    warehouseId: null, // انبار پیش‌فرض (ID=1)
+                    lowStockThreshold: 10,
+                    ct);
+
+                regularCount++;
+                _logger.LogDebug(
+                    "Initialized inventory for Regular Product Id={ProductId} ({Title}), Qty={Qty}",
+                    product.Id, product.Title, initialQty);
+            }
+            catch (Exception ex)
+            {
+                _logger.LogWarning(ex,
+                    "Failed to initialize inventory for Regular Product Id={ProductId} ({Title})",
+                    product.Id, product.Title);
+            }
+        }
+
+        // ── محصولات تخفیفی ──
+        var existingDiscountProductIds = await context.InventoryItems
+            .Where(i => !i.IsDeleted && i.DiscountProductId != null)
+            .Select(i => i.DiscountProductId!.Value)
+            .ToListAsync(ct);
+
+        var discountProducts = await context.DiscountProducts
+            .Where(p => !p.IsDeleted && !existingDiscountProductIds.Contains(p.Id))
+            .Select(p => new { p.Id, p.Title, p.RemainingCount })
+            .ToListAsync(ct);
+
+        _logger.LogInformation("Found {Count} discount products without inventory records", discountProducts.Count);
+
+        foreach (var product in discountProducts)
+        {
+            if (ct.IsCancellationRequested) break;
+
+            try
+            {
+                var initialQty = Math.Max(0, product.RemainingCount);
+
+                await inventoryService.InitializeInventoryAsync(
+                    product.Id,
+                    ProductType.DiscountProduct,
+                    initialQty,
+                    warehouseId: null,
+                    lowStockThreshold: 10,
+                    ct);
+
+                discountCount++;
+                _logger.LogDebug(
+                    "Initialized inventory for Discount Product Id={ProductId} ({Title}), Qty={Qty}",
+                    product.Id, product.Title, initialQty);
+            }
+            catch (Exception ex)
+            {
+                _logger.LogWarning(ex,
+                    "Failed to initialize inventory for Discount Product Id={ProductId} ({Title})",
+                    product.Id, product.Title);
+            }
+        }
+
+        return (regularCount, discountCount);
+    }
+}

src/CMSMicroservice.Infrastructure/ConfigureServices.cs

@@ -123,6 +123,9 @@ public static class ConfigureServices
         // Expire pending discount orders after 30 minutes
         services.AddHostedService<ExpirePendingOrdersService>();
         
+        // One-time: Initialize inventory records for existing products
+        services.AddHostedService<InventoryInitializerService>();
+        
         if (configuration.GetValue<bool>("UseInMemoryDatabase"))
         {
             services.AddDbContext<ApplicationDbContext>(options =>