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/mod.rs

@@ -1,11 +1,33 @@
-pub(crate) mod shared;
+mod announcements;
+pub(super) mod auth;
+mod billing;
+pub(super) mod endpoint;
+pub(super) mod features;
+mod model;
+pub(super) mod observability;
+pub(super) mod provider;
+mod system;
+mod users;
 
-pub(crate) mod auth;
-pub(crate) mod billing;
-pub(crate) mod endpoint;
-pub(crate) mod features;
-pub(crate) mod model;
-pub(crate) mod observability;
-pub(crate) mod provider;
-pub(crate) mod system;
-pub(crate) mod users;
+pub(super) mod request;
+pub(super) mod routes;
+mod shared;
+
+pub(crate) use self::auth::maybe_build_local_admin_security_response;
+pub(crate) use self::endpoint::build_admin_endpoint_health_status_payload;
+pub(crate) use self::features::maybe_build_local_admin_video_tasks_response;
+pub(crate) use self::observability::{
+    admin_stats_bad_request_response, list_usage_for_optional_range,
+    maybe_build_local_admin_usage_response, parse_bounded_u32, round_to, AdminStatsTimeRange,
+    AdminStatsUsageFilter,
+};
+pub(crate) use self::provider::oauth::errors::build_internal_control_error_response;
+pub(crate) use self::provider::ops::providers::actions::admin_provider_ops_local_action_response;
+pub(crate) use self::provider::pool_admin::maybe_build_local_admin_pool_response;
+pub(crate) use self::provider::{
+    maybe_build_local_admin_provider_oauth_response, maybe_build_local_admin_providers_response,
+};
+pub(crate) use self::request::{
+    AdminAppState, AdminRequestContext, AdminRouteRequest, AdminRouteResponse, AdminRouteResult,
+};
+pub(crate) use self::routes::maybe_build_local_admin_response;