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/Streaming/GetPublicEpg/GetPublicEpgQueryHandler.cs

@@ -56,7 +56,7 @@ public sealed class GetPublicEpgQueryHandler(IAppDbContext dbContext)
                 e.Id,
                 e.Title,
                 e.Overview,
-                e.StillPath,
+                e.StillImageId,
             })
             .ToListAsync(cancellationToken);
         var episodeByKey = episodes
@@ -82,7 +82,7 @@ public sealed class GetPublicEpgQueryHandler(IAppDbContext dbContext)
                     episode?.Id,
                     episode?.Title,
                     episode?.Overview,
-                    episode?.StillPath is not null
+                    episode?.StillImageId
                 );
             })
             .ToList();

backend/src/TeleWave.Application/Streaming/StreamingDtos.cs

@@ -22,7 +22,7 @@ public sealed record PublicEpgEntryDto(
     Guid? EpisodeId,
     string? EpisodeTitle,
     string? EpisodeOverview,
-    bool EpisodeHasStill
+    Guid? EpisodeStillImageId
 );
 
 public sealed record LiveSegmentDto(Guid AssetId, int LocalIndex, bool Discontinuity);