refactor: 大规模模块拆分与代码精简，新增 ai-pipeline/data-contracts 独立 crate

- 新增 aether-ai-pipeline 和 aether-data-contracts crate，将 pipeline 逻辑与数据契约从 gateway 中解耦
- 重构 admin handlers：拆分单体模块为 auth/billing/endpoint/features/model/observability/provider/system 等独立子模块
- 合并 chat/cli 重复代码路径：精简 conversion、finalize、planner 中的 sync/chat/cli 分支
- 重构 scheduler/executor/data 层，引入 facade 模式降低模块间耦合
- 移除冗余的 intent 模块，将 plan_fallback/policy/stream_path/sync_path 迁移至 executor
- 前端适配：调整 admin API 调用和 provider 模型测试对话框

apps/aether-gateway/src/tests/control/admin/usage.rs

@@ -1,8 +1,9 @@
 use std::sync::{Arc, Mutex};
 
 use aether_data::repository::provider_catalog::InMemoryProviderCatalogReadRepository;
-use aether_data::repository::usage::{InMemoryUsageReadRepository, StoredRequestUsageAudit};
+use aether_data::repository::usage::InMemoryUsageReadRepository;
 use aether_data::repository::users::{InMemoryUserReadRepository, StoredUserSummary};
+use aether_data_contracts::repository::usage::StoredRequestUsageAudit;
 use axum::body::{Body, Bytes};
 use axum::routing::{any, get, post};
 use axum::{extract::Request, Router};
@@ -20,7 +21,7 @@ use crate::constants::{
 };
 use crate::control::resolve_public_request_context;
 use crate::data::GatewayDataState;
-use crate::handlers::admin::maybe_build_local_admin_usage_response;
+use crate::handlers::admin::observability::maybe_build_local_admin_usage_response;
 
 const ADMIN_USAGE_DATA_UNAVAILABLE_DETAIL: &str = "Admin usage data unavailable";
 const DAY_1_UNIX_SECS: i64 = 1_711_000_000;