Add media image handling and related endpoints

- Introduced `MediaImage` entity to manage images for markdown in instructions and news.
- Updated `IAppDbContext` and `AppDbContext` to include `MediaImages` DbSet.
- Implemented `DeleteMediaImageFilesAsync` method in `FactoryResetCommandHandler` to remove media images during factory reset.
- Added new API endpoints for uploading and retrieving media images, enhancing markdown support.
- Updated frontend components to utilize the new `MarkdownEditor` for image uploads in instructions and news.
- Enhanced documentation to reflect the new media handling features and API specifications.

backend/tests/PnvPanel.Application.Tests/Media/UploadMediaImageCommandHandlerTests.cs

@@ -0,0 +1,95 @@
+using NSubstitute;
+using PnvPanel.Application.Admin.Media;
+using PnvPanel.Application.Common.Interfaces;
+using PnvPanel.Application.Media;
+using PnvPanel.Application.Tests.TestSupport;
+using Xunit;
+
+namespace PnvPanel.Application.Tests.Media;
+
+public class UploadMediaImageCommandHandlerTests
+{
+    private readonly IFileStorage _fileStorage = Substitute.For<IFileStorage>();
+
+    private static MediaImageUpload Upload(
+        string contentType = "image/png",
+        long sizeBytes = 1024,
+        string fileName = "screenshot.png"
+    ) => new(new MemoryStream([1, 2, 3]), fileName, contentType, sizeBytes);
+
+    [Fact]
+    public async Task Handle_WithValidImage_SavesFileAndRow()
+    {
+        using var dbContext = InMemoryDbContextFactory.Create();
+        var adminId = Guid.NewGuid();
+        _fileStorage
+            .SaveAsync(Arg.Any<Stream>(), Arg.Any<CancellationToken>())
+            .Returns("stored-name");
+
+        var handler = new UploadMediaImageCommandHandler(
+            dbContext,
+            _fileStorage,
+            FakeCurrentUser.Authenticated(adminId, "admin")
+        );
+
+        var result = await handler.Handle(
+            new UploadMediaImageCommand(Upload()),
+            CancellationToken.None
+        );
+        await dbContext.SaveChangesAsync(CancellationToken.None);
+
+        Assert.True(result.IsSuccess);
+        Assert.Equal("screenshot.png", result.Value.FileName);
+        var image = Assert.Single(dbContext.MediaImages);
+        Assert.Equal("stored-name", image.StoredFileName);
+        Assert.Equal(adminId, image.UploadedBy);
+    }
+
+    [Fact]
+    public async Task Handle_WithUnsupportedContentType_ReturnsValidationErrorWithoutSaving()
+    {
+        using var dbContext = InMemoryDbContextFactory.Create();
+
+        var handler = new UploadMediaImageCommandHandler(
+            dbContext,
+            _fileStorage,
+            FakeCurrentUser.Authenticated(Guid.NewGuid())
+        );
+
+        var result = await handler.Handle(
+            new UploadMediaImageCommand(Upload(contentType: "image/svg+xml", fileName: "x.svg")),
+            CancellationToken.None
+        );
+
+        Assert.False(result.IsSuccess);
+        Assert.Equal(MediaErrors.UnsupportedImageType, result.Error);
+        Assert.Empty(dbContext.MediaImages);
+        await _fileStorage
+            .DidNotReceive()
+            .SaveAsync(Arg.Any<Stream>(), Arg.Any<CancellationToken>());
+    }
+
+    /// <summary>Лимит продублирован из MediaImageValidation — она internal (см. Application).</summary>
+    private const long MaxSizeBytes = 5 * 1024 * 1024;
+
+    [Fact]
+    public async Task Handle_WithTooLargeImage_ReturnsValidationError()
+    {
+        using var dbContext = InMemoryDbContextFactory.Create();
+
+        var handler = new UploadMediaImageCommandHandler(
+            dbContext,
+            _fileStorage,
+            FakeCurrentUser.Authenticated(Guid.NewGuid())
+        );
+
+        var result = await handler.Handle(
+            new UploadMediaImageCommand(Upload(sizeBytes: MaxSizeBytes + 1)),
+            CancellationToken.None
+        );
+
+        Assert.False(result.IsSuccess);
+        Assert.Equal(MediaErrors.ImageTooLarge, result.Error);
+        Assert.Empty(dbContext.MediaImages);
+    }
+}