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/maintenance/runtime/schedule.rs

@@ -1,5 +1,6 @@
 use std::time::Duration;
 
+use aether_data_contracts::DataLayerError;
 use chrono::{DateTime, Datelike, TimeZone, Timelike, Utc, Weekday};
 use chrono_tz::Tz;
 use tracing::warn;
@@ -15,7 +16,7 @@ use super::{
 
 pub(super) async fn provider_checkin_schedule(
     data: &GatewayDataState,
-) -> Result<(u32, u32), aether_data::DataLayerError> {
+) -> Result<(u32, u32), DataLayerError> {
     let configured =
         system_config_string(data, "provider_checkin_time", PROVIDER_CHECKIN_DEFAULT_TIME).await?;
     Ok(parse_hhmm_time(&configured).unwrap_or_else(|| {