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/provider/request_context.py

@@ -0,0 +1,49 @@
+"""Per-request context shared across layers.
+
+We use `contextvars` so the transport layer (URL builder) can pass small bits of
+state to the handler layer without changing existing return types.
+
+This is intentionally minimal; only add fields that are safe and cheap to carry
+per request.
+"""
+
+from __future__ import annotations
+
+import contextvars
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from src.services.provider.fingerprint import FingerprintProfile
+
+_selected_base_url: contextvars.ContextVar[str | None] = contextvars.ContextVar(
+    "provider_selected_base_url",
+    default=None,
+)
+_current_fingerprint: contextvars.ContextVar[FingerprintProfile | None] = contextvars.ContextVar(
+    "provider_current_fingerprint",
+    default=None,
+)
+
+
+def set_selected_base_url(url: str | None) -> None:
+    _selected_base_url.set(url)
+
+
+def get_selected_base_url() -> str | None:
+    return _selected_base_url.get()
+
+
+def set_current_fingerprint(fp: FingerprintProfile | None) -> None:
+    _current_fingerprint.set(fp)
+
+
+def get_current_fingerprint() -> FingerprintProfile | None:
+    return _current_fingerprint.get()
+
+
+__all__ = [
+    "get_current_fingerprint",
+    "get_selected_base_url",
+    "set_current_fingerprint",
+    "set_selected_base_url",
+]