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/lib.rs

@@ -24,7 +24,9 @@
     clippy::useless_concat
 )]
 
-pub(crate) mod ai_pipeline;
+mod admin_api;
+mod ai_pipeline;
+mod ai_pipeline_api;
 mod api;
 mod async_task;
 mod audit;
@@ -57,7 +59,7 @@ mod usage;
 mod video_tasks;
 mod wallet_runtime;
 
-pub(crate) use self::ai_pipeline::contracts::{
+pub(crate) use self::ai_pipeline_api::{
     GatewayControlPlanRequest, GatewayControlPlanResponse, GatewayControlSyncDecisionResponse,
     EXECUTION_RUNTIME_STREAM_DECISION_ACTION, EXECUTION_RUNTIME_SYNC_DECISION_ACTION,
     GEMINI_FILES_DOWNLOAD_PLAN_KIND, OPENAI_VIDEO_CONTENT_PLAN_KIND,