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/tests/video/mod.rs

@@ -0,0 +1,32 @@
+use std::sync::{Arc, Mutex};
+
+use aether_data::repository::video_tasks::{
+    InMemoryVideoTaskRepository, UpsertVideoTask, VideoTaskLookupKey, VideoTaskReadRepository,
+    VideoTaskWriteRepository,
+};
+use axum::body::{to_bytes, Body, Bytes};
+use axum::response::Response;
+use axum::routing::any;
+use axum::{extract::Request, Json, Router};
+use http::header::{HeaderName, HeaderValue};
+use http::StatusCode;
+use serde_json::json;
+
+use crate::constants::{
+    CONTROL_EXECUTED_HEADER, CONTROL_EXECUTE_FALLBACK_HEADER, EXECUTION_PATH_HEADER,
+    TRACE_ID_HEADER,
+};
+
+use super::{
+    build_router, build_router_with_state, build_state_with_execution_runtime_override,
+    start_server, AppState, VideoTaskTruthSourceMode,
+};
+
+mod data_read;
+mod gemini_sync_create;
+mod gemini_sync_task;
+mod openai_sync_create;
+mod openai_sync_task;
+mod registry_poller;
+mod routing;
+mod stream;