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/usage/mod.rs

@@ -1,22 +1,15 @@
-mod bundle;
 mod config;
-mod event;
-mod http;
+pub(crate) mod event;
+pub(crate) mod http;
 mod queue;
-mod read;
-mod reporting;
+pub(crate) mod reporting;
 mod runtime;
 mod worker;
-mod write;
+pub(crate) mod write;
 
-pub(crate) use bundle::{read_request_audit_bundle, RequestAuditBundle};
 pub use config::UsageRuntimeConfig;
-pub(crate) use event::{UsageEvent, UsageEventData, UsageEventType};
-pub(crate) use http::{get_request_audit_bundle, get_request_usage_audit};
-pub(crate) use read::{read_request_usage_audit, RequestUsageAudit};
 pub(crate) use reporting::{
-    spawn_sync_report, store_local_gemini_file_mapping, submit_stream_report, submit_sync_report,
-    GatewayStreamReportRequest, GatewaySyncReportRequest,
+    spawn_sync_report, submit_stream_report, submit_sync_report, GatewayStreamReportRequest,
+    GatewaySyncReportRequest,
 };
 pub(crate) use runtime::UsageRuntime;
-pub(crate) use write::build_upsert_usage_record_from_event;