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/MediaImageUpload.cs

@@ -0,0 +1,10 @@
+namespace PnvPanel.Application.Media;
+
+/// <summary>Картинка на входе команды — Api-слой парсит multipart и передаёт сюда открытый поток;
+/// Application не знает про HTTP/IFormFile.</summary>
+public sealed record MediaImageUpload(
+    Stream Content,
+    string FileName,
+    string ContentType,
+    long SizeBytes
+);