refactor: 大规模模块拆分与重组，新增 aether-admin crate

- 新建独立 aether-admin crate 承载 admin 相关共享契约与纯辅助函数
- 拆分 ai_pipeline 下 kiro/private_envelope/conversion/planner 等大文件为子模块目录
- 重组 admin handlers 各业务域（billing/oauth/provider/system/users 等）为目录结构，移除 shared.rs/builders.rs 等反模式
- 移除 ai_pipeline runtime adapters 旧实现（claude/openai/gemini/kiro/vertex/antigravity 等），改由 provider transport 统一承载
- 移除 control_facade/execution_facade/auth_snapshot_facade 等冗余 facade 层
- 拆分 query/billing 与 query/monitoring 模块、state/runtime/payments 与 security 模块
- 扩展架构测试覆盖 admin_billing/admin_model/admin_users 等新模块
- 删除 docs/architecture/refactor-execution-plan.md 已完成的执行计划文档

apps/aether-gateway/src/usage/config.rs

@@ -1 +0,0 @@
-pub use aether_usage_runtime::UsageRuntimeConfig;

apps/aether-gateway/src/usage/event.rs

@@ -1,3 +0,0 @@
-pub use aether_usage_runtime::{
-    now_ms, UsageEvent, UsageEventData, UsageEventType, USAGE_EVENT_VERSION,
-};

apps/aether-gateway/src/usage/mod.rs

@@ -1,15 +1,14 @@
-mod config;
-pub(crate) mod event;
 pub(crate) mod http;
-mod queue;
 pub(crate) mod reporting;
-mod runtime;
 mod worker;
 pub(crate) mod write;
 
-pub use config::UsageRuntimeConfig;
+pub(crate) use aether_usage_runtime::UsageRuntime;
+pub use aether_usage_runtime::UsageRuntimeConfig;
+pub(crate) use aether_usage_runtime::{
+    now_ms, UsageEvent, UsageEventData, UsageEventType, UsageQueue, USAGE_EVENT_VERSION,
+};
 pub(crate) use reporting::{
     spawn_sync_report, submit_stream_report, submit_sync_report, GatewayStreamReportRequest,
     GatewaySyncReportRequest,
 };
-pub(crate) use runtime::UsageRuntime;

apps/aether-gateway/src/usage/queue.rs

@@ -1 +0,0 @@
-pub use aether_usage_runtime::UsageQueue;

apps/aether-gateway/src/usage/runtime.rs

@@ -1 +0,0 @@
-pub(crate) use aether_usage_runtime::UsageRuntime;

apps/aether-gateway/src/usage/worker.rs

@@ -9,7 +9,7 @@ mod tests {
 
     use super::write_event_record;
     use crate::data::GatewayDataState;
-    use crate::usage::event::{UsageEvent, UsageEventData, UsageEventType};
+    use crate::usage::{UsageEvent, UsageEventData, UsageEventType};
 
     #[tokio::test]
     async fn worker_writes_usage_record_from_terminal_event() {