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。

crates/aether-model-fetch/src/lib.rs

@@ -0,0 +1,17 @@
+mod association_sync;
+mod config;
+mod logic;
+mod transport;
+
+pub use association_sync::{
+    sync_provider_model_whitelist_associations, ModelFetchAssociationStore,
+};
+pub use config::{
+    model_fetch_interval_minutes, model_fetch_startup_delay_seconds, model_fetch_startup_enabled,
+};
+pub use logic::{
+    aggregate_models_for_cache, apply_model_filters, build_models_fetch_url,
+    endpoint_supports_rust_models_fetch, extract_error_message, json_string_list,
+    parse_models_response, select_models_fetch_endpoint, ModelFetchRunSummary, ModelsFetchSuccess,
+};
+pub use transport::{build_models_fetch_execution_plan, ModelFetchTransportRuntime};