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/handlers/admin/observability/routes.rs

@@ -0,0 +1,34 @@
+use super::{monitoring, stats, usage};
+use crate::handlers::admin::request::{AdminRouteRequest, AdminRouteResult};
+
+pub(crate) async fn maybe_build_local_admin_observability_response(
+    request: AdminRouteRequest<'_>,
+) -> AdminRouteResult {
+    if let Some(response) =
+        stats::maybe_build_local_admin_stats_response(&request.state(), &request.request_context())
+            .await?
+    {
+        return Ok(Some(response));
+    }
+
+    if let Some(response) = monitoring::maybe_build_local_admin_monitoring_response(
+        &request.state(),
+        &request.request_context(),
+    )
+    .await?
+    {
+        return Ok(Some(response));
+    }
+
+    if let Some(response) = usage::maybe_build_local_admin_usage_response(
+        &request.state(),
+        &request.request_context(),
+        request.request_body(),
+    )
+    .await?
+    {
+        return Ok(Some(response));
+    }
+
+    Ok(None)
+}