wire gap_filler into schedule engine, API, MCP

crates/presentation/src/handlers/channels.rs

@@ -71,6 +71,7 @@ pub async fn update_channel(
         schedule_config: req.schedule_config.map(Into::into),
         rotation_policy: req.rotation_policy,
         auto_schedule: req.auto_schedule,
+        gap_filler: req.gap_filler,
     };
     let channel = application::channels::update::execute(&state.channel_command_deps, cmd).await?;
     Ok(Json(ChannelResponse::from(channel)))