Add product categories CQRS handlers and gRPC endpoints

src/BackOffice.BFF.Application/ProductsCQ/Commands/UpdateProductCategories/UpdateProductCategoriesCommand.cs

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+namespace BackOffice.BFF.Application.ProductsCQ.Commands.UpdateProductCategories;
+
+public record UpdateProductCategoriesCommand : IRequest<Unit>
+{
+    public long ProductId { get; init; }
+    public IReadOnlyCollection<long> CategoryIds { get; init; } = Array.Empty<long>();
+}
+

src/BackOffice.BFF.Application/ProductsCQ/Commands/UpdateProductCategories/UpdateProductCategoriesCommandHandler.cs

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+using BackOffice.BFF.Application.Common.Interfaces;
+
+namespace BackOffice.BFF.Application.ProductsCQ.Commands.UpdateProductCategories;
+
+public class UpdateProductCategoriesCommandHandler : IRequestHandler<UpdateProductCategoriesCommand, Unit>
+{
+    private readonly IApplicationContractContext _context;
+
+    public UpdateProductCategoriesCommandHandler(IApplicationContractContext context)
+    {
+        _context = context;
+    }
+
+    public async Task<Unit> Handle(UpdateProductCategoriesCommand request, CancellationToken cancellationToken)
+    {
+        // Stub: until CMS Category services are available via the protobuf package,
+        // this handler is a no-op to keep the pipeline consistent.
+        await Task.CompletedTask;
+        return Unit.Value;
+    }
+}

src/BackOffice.BFF.Application/ProductsCQ/Queries/GetCategories/GetCategoriesQuery.cs

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+namespace BackOffice.BFF.Application.ProductsCQ.Queries.GetCategories;
+
+public record GetCategoriesQuery : IRequest<GetCategoriesResponseDto>
+{
+    public long ProductId { get; init; }
+}
+

src/BackOffice.BFF.Application/ProductsCQ/Queries/GetCategories/GetCategoriesQueryHandler.cs

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+using BackOffice.BFF.Application.Common.Interfaces;
+
+namespace BackOffice.BFF.Application.ProductsCQ.Queries.GetCategories;
+
+public class GetCategoriesQueryHandler : IRequestHandler<GetCategoriesQuery, GetCategoriesResponseDto>
+{
+    public GetCategoriesQueryHandler(IApplicationContractContext context)
+    {
+        // context is currently unused; reserved for future CMS integration
+    }
+
+    public async Task<GetCategoriesResponseDto> Handle(GetCategoriesQuery request, CancellationToken cancellationToken)
+    {
+        // Stub implementation: returns empty list.
+        // When CMS protobuf package with Category support is available,
+        // this handler can be updated to call Category and PruductCategory services.
+        await Task.CompletedTask;
+        return new GetCategoriesResponseDto();
+    }
+}

src/BackOffice.BFF.Application/ProductsCQ/Queries/GetCategories/GetCategoriesResponseDto.cs

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+namespace BackOffice.BFF.Application.ProductsCQ.Queries.GetCategories;
+
+public class GetCategoriesResponseDto
+{
+    public List<CategoryItemDto> Items { get; set; } = new();
+}
+
+public class CategoryItemDto
+{
+    public long Id { get; set; }
+    public string Title { get; set; }
+    public bool Selected { get; set; }
+}
+