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/handlers/shared/external_models.rs

@@ -0,0 +1,19 @@
+pub(crate) const OFFICIAL_EXTERNAL_MODEL_PROVIDERS: &[&str] = &[
+    "anthropic",
+    "openai",
+    "google",
+    "google-vertex",
+    "azure",
+    "amazon-bedrock",
+    "xai",
+    "meta",
+    "deepseek",
+    "mistral",
+    "cohere",
+    "zhipuai",
+    "alibaba",
+    "minimax",
+    "moonshot",
+    "baichuan",
+    "ai21",
+];