wire gap_filler into schedule engine, API, MCP

crates/application/src/channels/update.rs

@@ -35,6 +35,9 @@ pub async fn execute(deps: &ChannelCommandDeps, cmd: UpdateChannelCommand) -> Do
     if let Some(auto) = cmd.auto_schedule {
         channel.set_auto_schedule(auto);
     }
+    if let Some(gap_filler) = cmd.gap_filler {
+        channel.set_gap_filler(gap_filler);
+    }
 
     deps.channel_command.save(&channel).await?;