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/auth/oauth/models.py

@@ -0,0 +1,31 @@
+from dataclasses import dataclass
+from typing import Any
+
+
+@dataclass(frozen=True)
+class OAuthToken:
+    access_token: str
+    token_type: str = "bearer"
+    refresh_token: str | None = None
+    expires_in: int | None = None
+    id_token: str | None = None
+    scope: str | None = None
+    raw: dict[str, Any] | None = None
+
+
+@dataclass(frozen=True)
+class OAuthUserInfo:
+    id: str
+    username: str | None = None
+    email: str | None = None
+    email_verified: bool | None = None
+    raw: dict[str, Any] | None = None
+
+
+class OAuthFlowError(Exception):
+    """用于 OAuth 流程的可控错误（会映射到 error_code）。"""
+
+    def __init__(self, error_code: str, detail: str = ""):
+        super().__init__(error_code)
+        self.error_code = error_code
+        self.detail = detail