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/scheduler/mod.rs

@@ -1,6 +1,7 @@
 mod candidate;
 mod failover;
 mod health;
+mod request_candidate_state;
 mod request_candidates;
 mod route;
 
@@ -10,6 +11,9 @@ pub(crate) use candidate::{
     read_cached_scheduler_affinity_target, read_minimal_candidate_selection,
     GatewayMinimalCandidateSelectionCandidate,
 };
+pub(crate) use candidate::{
+    SchedulerCandidateSelectionRowSource, SchedulerRuntimeState,
+};
 pub(crate) use failover::{
     resolve_core_stream_direct_finalize_report_kind,
     resolve_core_stream_error_finalize_report_kind, resolve_core_sync_error_finalize_report_kind,
@@ -22,6 +26,7 @@ pub(crate) use health::{
     is_provider_key_circuit_open, provider_key_health_score, provider_key_rpm_allows_request_since,
     PROVIDER_KEY_RPM_WINDOW_SECS,
 };
+pub(crate) use request_candidate_state::SchedulerRequestCandidateRuntimeState;
 pub(crate) use request_candidates::{
     current_unix_secs, ensure_execution_request_candidate_slot, execution_error_details,
     record_local_request_candidate_status, record_report_request_candidate_status,