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/upstream_headers.py

@@ -0,0 +1,59 @@
+"""Provider-specific upstream request header hooks."""
+
+from __future__ import annotations
+
+from collections.abc import Mapping
+from typing import Any, Callable
+
+from src.core.provider_types import normalize_provider_type
+from src.services.provider.envelope import ensure_providers_bootstrapped
+
+UpstreamHeadersHookFn = Callable[..., dict[str, str]]
+
+_hooks: dict[tuple[str, str], UpstreamHeadersHookFn] = {}
+
+
+def register_upstream_headers_hook(
+    provider_type: str,
+    endpoint_sig: str,
+    hook: UpstreamHeadersHookFn,
+) -> None:
+    """Register a provider-specific extra upstream headers builder."""
+    pt = normalize_provider_type(provider_type)
+    sig = str(endpoint_sig or "").strip().lower()
+    if not pt or not sig:
+        return
+    _hooks[(pt, sig)] = hook
+
+
+def build_upstream_extra_headers(
+    *,
+    provider_type: str | None,
+    endpoint_sig: str | None,
+    request_body: Any,
+    original_headers: Mapping[str, Any] | None,
+    decrypted_auth_config: dict[str, Any] | None,
+) -> dict[str, str]:
+    """Build provider-specific extra upstream headers for the current request."""
+    pt = normalize_provider_type(provider_type)
+    sig = str(endpoint_sig or "").strip().lower()
+    if not pt or not sig:
+        return {}
+
+    ensure_providers_bootstrapped(provider_types=[pt])
+    hook = _hooks.get((pt, sig))
+    if hook is None:
+        return {}
+
+    return hook(
+        request_body,
+        original_headers,
+        decrypted_auth_config=decrypted_auth_config,
+    )
+
+
+__all__ = [
+    "UpstreamHeadersHookFn",
+    "build_upstream_extra_headers",
+    "register_upstream_headers_hook",
+]