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/Configs/Rotate/RotateVpnConfigCommandHandlerTests.cs

@@ -17,6 +17,7 @@ using PnvPanel.Domain.Instructions;
 using PnvPanel.Domain.News;
 using PnvPanel.Domain.Nodes;
 using PnvPanel.Domain.Plans;
+using PnvPanel.Domain.Media;
 using PnvPanel.Domain.Pricing;
 using PnvPanel.Domain.Support;
 using PnvPanel.Domain.Telegram;
@@ -45,6 +46,7 @@ public sealed class ThrowingSaveDbContext(AppDbContext inner) : IAppDbContext
     public DbSet<TicketAttachment> TicketAttachments => inner.TicketAttachments;
     public DbSet<InstructionIntro> InstructionIntros => inner.InstructionIntros;
     public DbSet<InstructionTab> InstructionTabs => inner.InstructionTabs;
+    public DbSet<MediaImage> MediaImages => inner.MediaImages;
     public DbSet<PricingSettings> PricingSettings => inner.PricingSettings;
     public DbSet<PricingDiscountTier> PricingDiscountTiers => inner.PricingDiscountTiers;
     public DbSet<BillingSettings> BillingSettings => inner.BillingSettings;