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/services/payment/gateway/wechat.py

@@ -0,0 +1,21 @@
+from __future__ import annotations
+
+from typing import Any
+
+from src.services.payment.gateway.base import PaymentGateway
+
+
+class WeChatGateway(PaymentGateway):
+    payment_method = "wechat"
+    display_name = "微信支付"
+
+    def create_checkout_payload(self, *, order: Any) -> dict[str, Any]:
+        gateway_order_id = getattr(order, "gateway_order_id", None) or f"wx_{order.order_no}"
+        return {
+            "gateway": self.payment_method,
+            "display_name": self.display_name,
+            "gateway_order_id": gateway_order_id,
+            "payment_url": f"/pay/mock/wechat/{order.order_no}",
+            "qr_code": f"mock://wechat/{order.order_no}",
+            "expires_at": getattr(order, "expires_at", None),
+        }