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/handlers/admin/models/write.rs

@@ -5,11 +5,11 @@ use super::{
     normalize_optional_price, normalize_required_trimmed_string,
     resolve_admin_global_model_by_id_or_err,
 };
-use crate::gateway::handlers::{
+use crate::handlers::{
     AdminGlobalModelCreateRequest, AdminGlobalModelUpdateRequest, AdminImportProviderModelsRequest,
     AdminProviderModelCreateRequest, AdminProviderModelUpdateRequest,
 };
-use crate::gateway::AppState;
+use crate::AppState;
 use aether_data::repository::global_models::{
     AdminProviderModelListQuery, CreateAdminGlobalModelRecord, StoredAdminGlobalModel,
     StoredAdminProviderModel, UpdateAdminGlobalModelRecord, UpsertAdminProviderModelRecord,