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/Cargo.toml

@@ -7,11 +7,13 @@ repository.workspace = true
 description = "Rust ingress gateway for Aether phase 3a transparent proxy"
 
 [dependencies]
+aether-ai-pipeline.workspace = true
 aether-billing.workspace = true
 aether-cache.workspace = true
 aether-contracts.workspace = true
 aether-crypto.workspace = true
 aether-data.workspace = true
+aether-data-contracts.workspace = true
 aether-http.workspace = true
 aether-model-fetch.workspace = true
 aether-provider-transport.workspace = true
@@ -42,6 +44,7 @@ reqwest.workspace = true
 rustls.workspace = true
 serde.workspace = true
 serde_json.workspace = true
+sha1 = "0.10"
 sha2.workspace = true
 sqlx.workspace = true
 thiserror.workspace = true