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/request/executor_plan.py

@@ -0,0 +1,41 @@
+"""
+旧 executor 命名兼容入口。
+
+当前主实现已经迁到 `execution_runtime_plan.py`。
+"""
+
+from src.services.request.execution_runtime_plan import (
+    ExecutionPlan,
+    ExecutionPlanBody,
+    ExecutionPlanTimeouts,
+    ExecutionProxySnapshot,
+    PreparedExecutionPlan,
+    build_execution_plan_body,
+    build_proxy_snapshot,
+    is_remote_contract_eligible,
+    is_remote_execution_runtime_contract_eligible,
+    is_remote_execution_runtime_proxy_supported,
+    is_remote_proxy_supported,
+    should_bypass_remote_execution_runtime,
+    should_bypass_remote_execution_runtime_url,
+    should_bypass_remote_executor,
+    should_bypass_remote_executor_url,
+)
+
+__all__ = [
+    "ExecutionPlan",
+    "ExecutionPlanBody",
+    "ExecutionPlanTimeouts",
+    "ExecutionProxySnapshot",
+    "PreparedExecutionPlan",
+    "build_execution_plan_body",
+    "build_proxy_snapshot",
+    "is_remote_contract_eligible",
+    "is_remote_execution_runtime_contract_eligible",
+    "is_remote_execution_runtime_proxy_supported",
+    "is_remote_proxy_supported",
+    "should_bypass_remote_execution_runtime",
+    "should_bypass_remote_execution_runtime_url",
+    "should_bypass_remote_executor",
+    "should_bypass_remote_executor_url",
+]