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/src/PnvPanel.Application/Media/GetImage/GetMediaImageQueryHandler.cs

@@ -0,0 +1,29 @@
+using Microsoft.EntityFrameworkCore;
+using LiteCqrs;
+using PnvPanel.Application.Common.Interfaces;
+using PnvPanel.Application.Common.Models;
+
+namespace PnvPanel.Application.Media.GetImage;
+
+public sealed class GetMediaImageQueryHandler(IAppDbContext dbContext, IFileStorage fileStorage)
+    : IQueryHandler<GetMediaImageQuery, Result<MediaImageContent>>
+{
+    public async Task<Result<MediaImageContent>> Handle(
+        GetMediaImageQuery query,
+        CancellationToken cancellationToken
+    )
+    {
+        var image = await dbContext
+            .MediaImages.AsNoTracking()
+            .FirstOrDefaultAsync(i => i.Id == query.ImageId, cancellationToken);
+
+        if (image is null)
+            return Result.Failure<MediaImageContent>(MediaErrors.NotFound);
+
+        var stream = await fileStorage.OpenReadAsync(image.StoredFileName, cancellationToken);
+        if (stream is null)
+            return Result.Failure<MediaImageContent>(MediaErrors.NotFound);
+
+        return Result.Success(new MediaImageContent(stream, image.ContentType, image.FileName));
+    }
+}