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/usage/service.py

@@ -0,0 +1,27 @@
+"""
+用量统计和配额管理服务
+"""
+
+from src.services.usage._types import UsageRecordParams
+from src.services.usage.active_requests import UsageActiveRequestsMixin
+from src.services.usage.cache_analysis import UsageCacheAnalysisMixin
+from src.services.usage.lifecycle import UsageLifecycleMixin
+from src.services.usage.pricing import UsagePricingMixin
+from src.services.usage.query import UsageQueryMixin
+from src.services.usage.recording import UsageRecordingMixin
+
+
+class UsageService(
+    UsagePricingMixin,
+    UsageRecordingMixin,
+    UsageLifecycleMixin,
+    UsageQueryMixin,
+    UsageActiveRequestsMixin,
+    UsageCacheAnalysisMixin,
+):
+    """用量统计服务"""
+
+    pass
+
+
+__all__ = ["UsageService", "UsageRecordParams"]