refactor: 拆分 gateway 单体为独立 crate，新增 systemd 部署方案

将 gateway 内部的 model-fetch、provider-transport、scheduler-core、
usage-runtime、video-tasks-core 模块提取为独立 crate；重构 gateway
内部模块结构（state/router/cache/data/query 等）；移除大量遗留模块
文件；新增 systemd 二进制部署骨架及相关文档；更新前端 usage 相关
API 和组件。

apps/aether-gateway/src/async_task/http/cancel.rs

@@ -3,7 +3,7 @@ use axum::response::IntoResponse;
 use axum::Json;
 use serde_json::{json, Map, Value};
 
-use crate::gateway::{AppState, GatewayError};
+use crate::{AppState, GatewayError};
 
 use super::super::finalize_video_task_if_terminal;
 use super::super::read_video_task_detail;
@@ -126,15 +126,19 @@ async fn execute_video_task_cancel_plan(
     trace_id: &str,
     plan: aether_contracts::ExecutionPlan,
 ) -> Result<(), axum::response::Response> {
-    let result = crate::gateway::execute_execution_runtime_sync_plan(state, Some(trace_id), &plan)
-        .await
-        .map_err(|err| {
-            GatewayError::UpstreamUnavailable {
-                trace_id: trace_id.to_string(),
-                message: format!("{err:?}"),
-            }
-            .into_response()
-        })?;
+    let result = crate::execution_runtime::execute_execution_runtime_sync_plan(
+        state,
+        Some(trace_id),
+        &plan,
+    )
+    .await
+    .map_err(|err| {
+        GatewayError::UpstreamUnavailable {
+            trace_id: trace_id.to_string(),
+            message: format!("{err:?}"),
+        }
+        .into_response()
+    })?;
 
     if result.status_code >= 400 {
         let status = axum::http::StatusCode::from_u16(result.status_code)