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/common.py

@@ -0,0 +1,14 @@
+from __future__ import annotations
+
+import ipaddress
+
+from fastapi import HTTPException, Request
+
+
+def ensure_loopback(request: Request) -> None:
+    host = request.client.host if request.client else ""
+    try:
+        if not ipaddress.ip_address(host).is_loopback:
+            raise ValueError(host)
+    except ValueError as exc:
+        raise HTTPException(status_code=403, detail="loopback access only") from exc