Refactor bumper template background management: rename background upload endpoint to reflect new functionality, update related command and handler to use image ID instead of extension, and adjust data models to support image references. Remove obsolete background handling code and enhance UI components for improved image selection and display.

backend/src/TeleWave.Application/Library/GetShow/GetShowQueryHandler.cs

@@ -44,7 +44,7 @@ public sealed class GetShowQueryHandler(IAppDbContext dbContext)
                     e.Episode,
                     e.Title,
                     e.Overview,
-                    e.StillPath is not null,
+                    e.StillImageId,
                     e.AirDate
                 );
             })