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/executor/remote.rs

@@ -0,0 +1,27 @@
+use axum::body::Body;
+use axum::http::Response;
+
+use crate::control::GatewayControlDecision;
+use crate::{AppState, GatewayError};
+
+pub(crate) async fn maybe_execute_sync_via_remote_decision(
+    _state: &AppState,
+    _parts: &http::request::Parts,
+    _trace_id: &str,
+    _decision: &GatewayControlDecision,
+    _body_json: &serde_json::Value,
+    _plan_kind: &str,
+) -> Result<Option<Response<Body>>, GatewayError> {
+    Ok(None)
+}
+
+pub(crate) async fn maybe_execute_stream_via_remote_decision(
+    _state: &AppState,
+    _parts: &http::request::Parts,
+    _trace_id: &str,
+    _decision: &GatewayControlDecision,
+    _body_json: &serde_json::Value,
+    _plan_kind: &str,
+) -> Result<Option<Response<Body>>, GatewayError> {
+    Ok(None)
+}