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

crates/aether-billing/src/event_enrichment.rs

@@ -1,5 +1,5 @@
-use aether_data::repository::billing::StoredBillingModelContext;
-use aether_data::DataLayerError;
+use aether_data_contracts::repository::billing::StoredBillingModelContext;
+use aether_data_contracts::DataLayerError;
 use aether_usage_runtime::{UsageEvent, UsageEventType};
 use async_trait::async_trait;
 use serde_json::{Map, Value};
@@ -136,7 +136,7 @@ fn merge_billing_snapshot_metadata(
 
 #[cfg(test)]
 mod tests {
-    use aether_data::repository::billing::StoredBillingModelContext;
+    use aether_data_contracts::repository::billing::StoredBillingModelContext;
     use aether_usage_runtime::{UsageEvent, UsageEventData, UsageEventType};
     use async_trait::async_trait;
     use serde_json::json;
@@ -155,7 +155,8 @@ mod tests {
             _provider_id: &str,
             _provider_api_key_id: Option<&str>,
             _global_model_name: &str,
-        ) -> Result<Option<StoredBillingModelContext>, aether_data::DataLayerError> {
+        ) -> Result<Option<StoredBillingModelContext>, aether_data_contracts::DataLayerError>
+        {
             Ok(self.context.clone())
         }
     }