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/usage/mod.rs

@@ -0,0 +1,22 @@
+mod bundle;
+mod config;
+mod event;
+mod http;
+mod queue;
+mod read;
+mod reporting;
+mod runtime;
+mod worker;
+mod write;
+
+pub(crate) use bundle::{read_request_audit_bundle, RequestAuditBundle};
+pub use config::UsageRuntimeConfig;
+pub(crate) use event::{UsageEvent, UsageEventData, UsageEventType};
+pub(crate) use http::{get_request_audit_bundle, get_request_usage_audit};
+pub(crate) use read::{read_request_usage_audit, RequestUsageAudit};
+pub(crate) use reporting::{
+    spawn_sync_report, store_local_gemini_file_mapping, submit_stream_report, submit_sync_report,
+    GatewayStreamReportRequest, GatewaySyncReportRequest,
+};
+pub(crate) use runtime::UsageRuntime;
+pub(crate) use write::build_upsert_usage_record_from_event;