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/GetMediaImageQueryHandlerTests.cs

@@ -0,0 +1,76 @@
+using NSubstitute;
+using PnvPanel.Application.Common.Interfaces;
+using PnvPanel.Application.Media;
+using PnvPanel.Application.Media.GetImage;
+using PnvPanel.Application.Tests.TestSupport;
+using PnvPanel.Domain.Media;
+using Xunit;
+
+namespace PnvPanel.Application.Tests.Media;
+
+public class GetMediaImageQueryHandlerTests
+{
+    private readonly IFileStorage _fileStorage = Substitute.For<IFileStorage>();
+
+    [Fact]
+    public async Task Handle_WhenImageExists_ReturnsContentStream()
+    {
+        using var dbContext = InMemoryDbContextFactory.Create();
+        var image = MediaImage.Create("shot.png", "stored-name", "image/png", 10, Guid.NewGuid());
+        dbContext.MediaImages.Add(image);
+        await dbContext.SaveChangesAsync(CancellationToken.None);
+
+        _fileStorage
+            .OpenReadAsync("stored-name", Arg.Any<CancellationToken>())
+            .Returns(new MemoryStream([1, 2, 3]));
+
+        var handler = new GetMediaImageQueryHandler(dbContext, _fileStorage);
+
+        var result = await handler.Handle(
+            new GetMediaImageQuery(image.Id),
+            CancellationToken.None
+        );
+
+        Assert.True(result.IsSuccess);
+        Assert.Equal("image/png", result.Value.ContentType);
+        Assert.Equal("shot.png", result.Value.FileName);
+    }
+
+    [Fact]
+    public async Task Handle_WhenRowExistsButFileMissing_ReturnsNotFound()
+    {
+        using var dbContext = InMemoryDbContextFactory.Create();
+        var image = MediaImage.Create("shot.png", "stored-name", "image/png", 10, Guid.NewGuid());
+        dbContext.MediaImages.Add(image);
+        await dbContext.SaveChangesAsync(CancellationToken.None);
+
+        _fileStorage
+            .OpenReadAsync("stored-name", Arg.Any<CancellationToken>())
+            .Returns((Stream?)null);
+
+        var handler = new GetMediaImageQueryHandler(dbContext, _fileStorage);
+
+        var result = await handler.Handle(
+            new GetMediaImageQuery(image.Id),
+            CancellationToken.None
+        );
+
+        Assert.False(result.IsSuccess);
+        Assert.Equal(MediaErrors.NotFound, result.Error);
+    }
+
+    [Fact]
+    public async Task Handle_WhenImageUnknown_ReturnsNotFound()
+    {
+        using var dbContext = InMemoryDbContextFactory.Create();
+        var handler = new GetMediaImageQueryHandler(dbContext, _fileStorage);
+
+        var result = await handler.Handle(
+            new GetMediaImageQuery(Guid.NewGuid()),
+            CancellationToken.None
+        );
+
+        Assert.False(result.IsSuccess);
+        Assert.Equal(MediaErrors.NotFound, result.Error);
+    }
+}