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/state/mod.rs

@@ -0,0 +1,36 @@
+use super::error::GatewayError;
+use super::data::GatewayDataState;
+
+mod admin_types;
+mod app;
+mod cache;
+mod catalog;
+mod core;
+mod cors;
+mod integrations;
+mod oauth;
+mod runtime;
+#[cfg(test)]
+mod testing;
+mod types;
+mod video;
+
+pub(crate) use self::admin_types::{
+    AdminBillingCollectorRecord, AdminBillingCollectorWriteInput, AdminBillingPresetApplyResult,
+    AdminBillingRuleRecord, AdminBillingRuleWriteInput, AdminPaymentCallbackRecord,
+    AdminSecurityBlacklistEntry, AdminWalletPaymentOrderRecord, AdminWalletRefundRecord,
+    AdminWalletTransactionRecord,
+};
+pub use self::app::AppState;
+pub(crate) use self::cache::{
+    CachedProviderTransportSnapshot, AUTH_API_KEY_LAST_USED_MAX_ENTRIES,
+    AUTH_API_KEY_LAST_USED_TTL, PROVIDER_TRANSPORT_SNAPSHOT_CACHE_MAX_ENTRIES,
+    PROVIDER_TRANSPORT_SNAPSHOT_CACHE_TTL,
+};
+pub use self::cors::FrontdoorCorsConfig;
+pub(crate) use self::types::{
+    AdminWalletMutationOutcome, LocalExecutionRuntimeMissDiagnostic, LocalMutationOutcome,
+    LocalProviderDeleteTaskState,
+};
+use super::provider_transport::provider_transport_snapshot_looks_refreshed;
+pub(crate) use super::provider_transport::ProviderTransportSnapshotCacheKey;