refactor: 大规模模块拆分与重组，新增 aether-admin crate

- 新建独立 aether-admin crate 承载 admin 相关共享契约与纯辅助函数
- 拆分 ai_pipeline 下 kiro/private_envelope/conversion/planner 等大文件为子模块目录
- 重组 admin handlers 各业务域（billing/oauth/provider/system/users 等）为目录结构，移除 shared.rs/builders.rs 等反模式
- 移除 ai_pipeline runtime adapters 旧实现（claude/openai/gemini/kiro/vertex/antigravity 等），改由 provider transport 统一承载
- 移除 control_facade/execution_facade/auth_snapshot_facade 等冗余 facade 层
- 拆分 query/billing 与 query/monitoring 模块、state/runtime/payments 与 security 模块
- 扩展架构测试覆盖 admin_billing/admin_model/admin_users 等新模块
- 删除 docs/architecture/refactor-execution-plan.md 已完成的执行计划文档

apps/aether-gateway/src/handlers/admin/users/lifecycle/delete.rs

@@ -0,0 +1,66 @@
+use super::super::build_admin_users_bad_request_response;
+use super::support::admin_user_id_from_detail_path;
+use crate::handlers::admin::request::{AdminAppState, AdminRequestContext};
+use crate::handlers::admin::shared::attach_admin_audit_response;
+use crate::GatewayError;
+use axum::{
+    body::Body,
+    http,
+    response::{IntoResponse, Response},
+    Json,
+};
+use serde_json::json;
+
+pub(in super::super) async fn build_admin_delete_user_response(
+    state: &AdminAppState<'_>,
+    request_context: &AdminRequestContext<'_>,
+) -> Result<Response<Body>, GatewayError> {
+    let Some(user_id) = admin_user_id_from_detail_path(request_context.path()) else {
+        return Ok(build_admin_users_bad_request_response("缺少 user_id"));
+    };
+    let Some(user) = state.find_user_auth_by_id(&user_id).await? else {
+        return Ok((
+            http::StatusCode::NOT_FOUND,
+            Json(json!({ "detail": "用户不存在" })),
+        )
+            .into_response());
+    };
+
+    if user.role.eq_ignore_ascii_case("admin") && state.count_active_admin_users().await? <= 1 {
+        return Ok((
+            http::StatusCode::BAD_REQUEST,
+            Json(json!({ "detail": "不能删除最后一个管理员账户" })),
+        )
+            .into_response());
+    }
+    if state.count_user_pending_refunds(&user_id).await? > 0 {
+        return Ok((
+            http::StatusCode::BAD_REQUEST,
+            Json(json!({ "detail": "用户存在未完结退款，禁止删除" })),
+        )
+            .into_response());
+    }
+    if state.count_user_pending_payment_orders(&user_id).await? > 0 {
+        return Ok((
+            http::StatusCode::BAD_REQUEST,
+            Json(json!({ "detail": "用户存在未完结充值订单，禁止删除" })),
+        )
+            .into_response());
+    }
+
+    if !state.delete_local_auth_user(&user_id).await? {
+        return Ok((
+            http::StatusCode::NOT_FOUND,
+            Json(json!({ "detail": "用户不存在" })),
+        )
+            .into_response());
+    }
+
+    Ok(attach_admin_audit_response(
+        Json(json!({ "message": "用户删除成功" })).into_response(),
+        "admin_user_deleted",
+        "delete_user",
+        "user",
+        &user_id,
+    ))
+}