refactor: 移除 Python 后端源码，全面迁移至 Rust gateway 架构

- 删除全部 Python 源码 (src/) 及 Alembic 迁移脚本，归档至 _deprecated_py_src/
- 重构 Rust gateway ai_pipeline: 拆分 planner/finalize 模块，新增 contracts/adaptation 层
- 重组 handlers 模块为 admin/public/proxy/internal/shared 子模块结构
- 新增 executor 模块，引入 Rust 原生数据库迁移 (aether-data/migrations)
- 简化 CI/Docker 构建流程，移除 base image 二级构建，统一为单一 app image
- 移除 Python 相关基础设施文件 (entrypoint.sh, gunicorn_conf.py, Dockerfile.base)

apps/aether-gateway/src/execution_runtime/stream/execution.rs

@@ -26,6 +26,9 @@ use self::execution_failures::{
 use crate::gateway::ai_pipeline::runtime::{
     maybe_build_provider_private_stream_normalizer, normalize_provider_private_report_context,
 };
+use crate::gateway::api::response::{
+    attach_control_metadata_headers, build_client_response, build_client_response_from_parts,
+};
 use crate::gateway::constants::{CONTROL_CANDIDATE_ID_HEADER, CONTROL_REQUEST_ID_HEADER};
 use crate::gateway::execution_runtime::build_direct_execution_frame_stream;
 #[cfg(test)]
@@ -36,18 +39,15 @@ use crate::gateway::execution_runtime::submission::{
 use crate::gateway::execution_runtime::transport::{
     DirectSyncExecutionRuntime, DirectUpstreamStreamExecution,
 };
-use crate::gateway::request_candidates::{
+use crate::gateway::scheduler::{
     current_unix_secs as current_request_candidate_unix_secs,
     ensure_execution_request_candidate_slot, record_local_request_candidate_status,
-};
-use crate::gateway::scheduler::{
     resolve_core_stream_direct_finalize_report_kind,
     resolve_core_stream_error_finalize_report_kind, should_fallback_to_control_stream,
     should_retry_next_local_candidate_stream,
 };
 use crate::gateway::usage::submit_stream_report;
 use crate::gateway::{
-    attach_control_metadata_headers, build_client_response, build_client_response_from_parts,
     maybe_build_stream_response_rewriter, AppState, GatewayControlDecision, GatewayError,
     GatewayStreamReportRequest, GatewaySyncReportRequest, MAX_STREAM_PREFETCH_BYTES,
     MAX_STREAM_PREFETCH_FRAMES,