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/public/__init__.py

@@ -0,0 +1,24 @@
+"""Public-facing API routers.
+
+Keep the compatibility frontdoor surface explicit so Rust can take ownership of
+that manifest later without re-auditing the entire Python public app shell.
+"""
+
+from __future__ import annotations
+
+from fastapi import APIRouter
+
+from .support import python_public_support_router
+
+router = APIRouter()
+router.include_router(python_public_support_router)
+
+__all__ = ["frontdoor_compat_router", "python_public_support_router", "router"]
+
+
+def __getattr__(name: str) -> object:
+    if name == "frontdoor_compat_router":
+        from .compat import frontdoor_compat_router
+
+        return frontdoor_compat_router
+    raise AttributeError(f"module {__name__!r} has no attribute {name!r}")