feat: integrate PYMS payment gateway, add blog/sitepage/image services, local file manager

Payment Gateway:
- Add PYMSPaymentService: IPaymentGatewayService via gRPC to PYMS microservice
- Add ZarinPalPaymentService: direct ZarinPal integration (backup)
- Register 'pyms' payment provider in DI ConfigureServices
- Add PYMS proto files (pyms_transaction.proto, pyms_public_messages.proto)
- Fix VerifyDiscountWalletCharge: pass 'OK' as status instead of Authority
- Update appsettings: PaymentProvider=pyms, sandbox mode, merchant ID

Blog System:
- Add BlogCategory, BlogPost, BlogPostImage entities and CQRS
- Add proto files and gRPC services for blog management
- Add Mapster profiles for blog responses

Content Management:
- Add SitePage entity and CQRS for static pages
- Add proto and gRPC service for site pages

Image/File Management:
- Add LocalFileManager with disk storage + base64 serving + FMS fallback
- Add ImagePathResolverInterceptor for gRPC responses
- Add ImageResolverService for explicit image resolution
- Add UploadsController for public file serving with FMS fallback
- Add PaymentCallbackController for discount order payment callbacks

Database:
- Add blog and content entity migrations
- Remove ImagePath MaxLength constraints
- Remove old FileManagementService (replaced by LocalFileManager)

src/CMSMicroservice.Application/BlogPostCQ/Queries/GetBlogPostBySlug/GetBlogPostBySlugQuery.cs

@@ -0,0 +1,8 @@
+using MediatR;
+
+namespace CMSMicroservice.Application.BlogPostCQ.Queries.GetBlogPostBySlug;
+
+public class GetBlogPostBySlugQuery : IRequest<GetBlogPost.BlogPostDto>
+{
+    public string Slug { get; set; } = string.Empty;
+}

src/CMSMicroservice.Application/BlogPostCQ/Queries/GetBlogPostBySlug/GetBlogPostBySlugQueryHandler.cs

@@ -0,0 +1,58 @@
+using CMSMicroservice.Application.BlogPostCQ.Queries.GetBlogPost;
+using CMSMicroservice.Application.Common.Interfaces;
+using CMSMicroservice.Domain.Enums;
+using MediatR;
+using Microsoft.EntityFrameworkCore;
+
+namespace CMSMicroservice.Application.BlogPostCQ.Queries.GetBlogPostBySlug;
+
+public class GetBlogPostBySlugQueryHandler : IRequestHandler<GetBlogPostBySlugQuery, BlogPostDto>
+{
+    private readonly IApplicationDbContext _context;
+
+    public GetBlogPostBySlugQueryHandler(IApplicationDbContext context)
+    {
+        _context = context;
+    }
+
+    public async Task<BlogPostDto> Handle(GetBlogPostBySlugQuery request, CancellationToken cancellationToken)
+    {
+        var post = await _context.BlogPosts
+            .Include(x => x.BlogPostCategories).ThenInclude(x => x.BlogCategory)
+            .Include(x => x.BlogPostTags).ThenInclude(x => x.Tag)
+            .FirstOrDefaultAsync(x => x.Slug == request.Slug && !x.IsDeleted, cancellationToken)
+            ?? throw new KeyNotFoundException($"مقاله با نشانی '{request.Slug}' یافت نشد");
+
+        return new BlogPostDto
+        {
+            Id = post.Id,
+            Title = post.Title,
+            Slug = post.Slug,
+            Summary = post.Summary,
+            HtmlContent = post.HtmlContent,
+            FeaturedImagePath = post.FeaturedImagePath,
+            FeaturedImageThumbnailPath = post.FeaturedImageThumbnailPath,
+            Status = post.Status,
+            StatusName = GetBlogPostQueryHandler.GetStatusName(post.Status),
+            PublishedAt = post.PublishedAt,
+            ViewCount = post.ViewCount,
+            AuthorUserId = post.AuthorUserId,
+            IsFeatured = post.IsFeatured,
+            SortOrder = post.SortOrder,
+            Created = post.Created,
+            LastModified = post.LastModified,
+            Categories = post.BlogPostCategories.Select(c => new BlogPostCategoryDto
+            {
+                Id = c.BlogCategory.Id,
+                Title = c.BlogCategory.Title,
+                Slug = c.BlogCategory.Slug
+            }).ToList(),
+            Tags = post.BlogPostTags.Select(t => new BlogPostTagDto
+            {
+                Id = t.Tag.Id,
+                Title = t.Tag.Title,
+                Name = t.Tag.Name
+            }).ToList()
+        };
+    }
+}