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-data/src/repository/candidates/mod.rs

@@ -5,7 +5,9 @@ mod types;
 pub use memory::InMemoryRequestCandidateRepository;
 pub use sql::SqlxRequestCandidateReadRepository;
 pub use types::{
-    PublicHealthStatusCount, PublicHealthTimelineBucket, RequestCandidateReadRepository,
-    RequestCandidateRepository, RequestCandidateStatus, RequestCandidateWriteRepository,
+    build_decision_trace, derive_request_candidate_final_status, DecisionTrace,
+    DecisionTraceCandidate, PublicHealthStatusCount, PublicHealthTimelineBucket,
+    RequestCandidateFinalStatus, RequestCandidateReadRepository, RequestCandidateRepository,
+    RequestCandidateStatus, RequestCandidateTrace, RequestCandidateWriteRepository,
     StoredRequestCandidate, UpsertRequestCandidateRecord,
 };