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/task/video/facade.py

@@ -0,0 +1,38 @@
+from __future__ import annotations
+
+from typing import Any
+
+from src.services.task.core.schema import TaskStatusResult
+from src.services.task.video.operations import VideoTaskOperationsService
+
+
+class TaskVideoFacadeService:
+    """视频任务门面服务（向后兼容 TaskService 的视频公开方法）。"""
+
+    def __init__(self, video_ops: VideoTaskOperationsService) -> None:
+        self._video_ops = video_ops
+
+    async def poll(self, task_id: str, *, user_id: str) -> TaskStatusResult:
+        return await self._video_ops.poll(task_id, user_id=user_id)
+
+    async def poll_now(self, task_id: str, *, user_id: str) -> TaskStatusResult:
+        return await self._video_ops.poll_now(task_id, user_id=user_id)
+
+    async def cancel(
+        self,
+        task_id: str,
+        *,
+        user_id: str,
+        original_headers: dict[str, str] | None = None,
+    ) -> Any:
+        return await self._video_ops.cancel(
+            task_id,
+            user_id=user_id,
+            original_headers=original_headers,
+        )
+
+    async def finalize_video_task(self, task: Any) -> bool:
+        return await self._video_ops.finalize_video_task(task)
+
+    async def finalize(self, task_id: str) -> bool:
+        return await self._video_ops.finalize(task_id)