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/api/public/compat.py

@@ -0,0 +1,26 @@
+"""Rust-frontdoor-owned public compatibility route definitions."""
+
+from fastapi import APIRouter
+
+from .claude import router as claude_router
+from .gemini import router as gemini_router
+from .gemini_files import router as gemini_files_router
+from .openai import router as openai_router
+from .videos import router as videos_router
+
+
+def build_frontdoor_compat_router() -> APIRouter:
+    """Return public compat routes that Rust frontdoor owns at host level."""
+    compat_router = APIRouter()
+
+    compat_router.include_router(videos_router, tags=["Video Generation"])
+    compat_router.include_router(claude_router, tags=["Claude API"])
+    compat_router.include_router(openai_router)
+    compat_router.include_router(gemini_router, tags=["Gemini API"])
+    compat_router.include_router(gemini_files_router, tags=["Gemini Files API"])
+    return compat_router
+
+
+frontdoor_compat_router = build_frontdoor_compat_router()
+
+__all__ = ["build_frontdoor_compat_router", "frontdoor_compat_router"]