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)

_deprecated_py_src/api/internal/gateway_finalize.py

@@ -0,0 +1,41 @@
+"""Compatibility re-export layer for gateway finalize helpers."""
+
+from __future__ import annotations
+
+from .gateway_finalize_chat import (
+    _finalize_gateway_chat_sync,
+    _run_gateway_chat_sync_finalize_background,
+    _run_gateway_chat_sync_finalize_background_with_session,
+)
+from .gateway_finalize_cli import (
+    _finalize_gateway_cli_sync,
+    _run_gateway_cli_sync_finalize_background,
+    _run_gateway_cli_sync_finalize_background_with_session,
+)
+from .gateway_finalize_common import (
+    _build_gateway_embedded_error_payload,
+    _build_gateway_sync_error_payload,
+    _extract_gateway_report_body_bytes,
+    _extract_gateway_sync_error_message,
+    _finalize_gateway_sync_response,
+    _gateway_module,
+    _resolve_gateway_finalize_db,
+    _resolve_gateway_sync_error_status_code,
+)
+
+__all__ = [
+    "_build_gateway_embedded_error_payload",
+    "_build_gateway_sync_error_payload",
+    "_extract_gateway_report_body_bytes",
+    "_extract_gateway_sync_error_message",
+    "_finalize_gateway_chat_sync",
+    "_finalize_gateway_cli_sync",
+    "_finalize_gateway_sync_response",
+    "_gateway_module",
+    "_resolve_gateway_finalize_db",
+    "_resolve_gateway_sync_error_status_code",
+    "_run_gateway_chat_sync_finalize_background",
+    "_run_gateway_chat_sync_finalize_background_with_session",
+    "_run_gateway_cli_sync_finalize_background",
+    "_run_gateway_cli_sync_finalize_background_with_session",
+]