Add TV bumpers functionality: introduce configuration options for bumpers in .env.example and appsettings.json, enhance ChannelEndpoints to manage jingles and bumper assets, and update Channel and ScheduleEntry models to support bumper logic. Implement validation for bumper settings and integrate bumper handling in scheduling logic.

backend/src/TeleWave.Application/Broadcast/BumperBackground/SetBumperBackgroundCommand.cs

@@ -0,0 +1,7 @@
+using LiteCqrs;
+using TeleWave.Application.Common.Models;
+
+namespace TeleWave.Application.Broadcast.BumperBackground;
+
+/// <summary>Отметить, что для канала загружен фон заставки (файл уже сохранён хранилищем).</summary>
+public sealed record SetBumperBackgroundCommand(Guid ChannelId, string Extension) : ICommand<Result>;