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/scheduling/quota_skipper.py

@@ -0,0 +1,17 @@
+from __future__ import annotations
+
+from src.models.database import ProviderAPIKey
+from src.services.provider_keys.quota_reader import get_quota_reader
+
+
+def is_key_quota_exhausted(
+    provider_type: str | None,
+    key: ProviderAPIKey,
+    *,
+    model_name: str,
+) -> tuple[bool, str | None]:
+    """Check ProviderAPIKey.upstream_metadata quota and decide whether to skip."""
+
+    reader = get_quota_reader(provider_type, getattr(key, "upstream_metadata", None))
+    result = reader.is_exhausted(model_name)
+    return result.exhausted, result.reason