refactor: 移除独立 hub/proxy/executor/gateway crate，统一为 gateway tunnel 架构

- 删除 aether-hub、aether-proxy 独立项目及其 Dockerfile/配置
- 删除 crates/aether-executor 和 crates/aether-gateway 全部模块
- 新增 apps/ 目录作为应用入口
- 将 hub 概念重构为 gateway tunnel transport
- 将 executor 重构为 execution runtime
- 新增 tunnel.rs 合约定义和 testkit tunnel/execution_runtime 模块
- 更新 Python 服务层和测试适配新架构命名

apps/aether-gateway/src/maintenance/mod.rs

@@ -0,0 +1,10 @@
+mod runtime;
+
+pub(crate) use runtime::{
+    perform_provider_checkin_once, spawn_audit_cleanup_worker, spawn_db_maintenance_worker,
+    spawn_gemini_file_mapping_cleanup_worker, spawn_pending_cleanup_worker,
+    spawn_pool_monitor_worker, spawn_provider_checkin_worker,
+    spawn_request_candidate_cleanup_worker, spawn_stats_aggregation_worker,
+    spawn_stats_hourly_aggregation_worker, spawn_usage_cleanup_worker,
+    spawn_wallet_daily_usage_aggregation_worker, ProviderCheckinRunSummary,
+};