Implement BumperEndpoints and remove deprecated bumper-related functionality

Added new BumperEndpoints to the API for managing bumper templates and variants, enhancing the channel management capabilities. Removed outdated bumper-related commands and handlers from the application, streamlining the codebase and improving maintainability. Updated ChannelEndpoints to reflect these changes and ensure proper routing for the new endpoints.

backend/src/TeleWave.Application/Broadcast/Bumpers/CreateBumperTemplateCommandHandler.cs

@@ -0,0 +1,24 @@
+using LiteCqrs;
+using TeleWave.Application.Common.Interfaces;
+using TeleWave.Application.Common.Models;
+using TeleWave.Domain.Broadcast;
+
+namespace TeleWave.Application.Broadcast.Bumpers;
+
+public sealed class CreateBumperTemplateCommandHandler(IAppDbContext dbContext)
+    : ICommandHandler<CreateBumperTemplateCommand, Result<Guid>>
+{
+    private const string DefaultVariantName = "Текст 1";
+
+    public Task<Result<Guid>> Handle(
+        CreateBumperTemplateCommand command,
+        CancellationToken cancellationToken
+    )
+    {
+        var template = BumperTemplate.Create(command.Name, DefaultVariantName);
+        // Пустой блок в редакторе выглядит поломанным — новый начинается с «Сейчас / Далее».
+        template.Variants[0].SetLines(BumperTemplateLoader.DefaultLines());
+        dbContext.BumperTemplates.Add(template);
+        return Task.FromResult(Result.Success(template.Id));
+    }
+}