feat: M1 — 立项→写章草稿(SSE)→自动保存；连一家 provider

- 薄自建 LLM 网关：OpenAI 兼容适配器(DeepSeek) + instructor 结构化输出 + usage_ledger 记账 + 档位路由
- 记忆服务 assemble：确定性选择(显式+主角+近况) + 渲染卡 + 缓存断点(中性文本)
- LangGraph 写章节点 + Postgres checkpointer + SSE 归一(token/done/error)
- API：立项 + 写章 draft(SSE) + PUT 自动保存 + 提供商凭据(Fernet 加密/测试连接)
- 前端：AppShell + 作品库 + 5 步立项向导 + 写作工作台(流式打字机+自动保存) + 设置页
- M1 E2E：真实 DB + mock 网关零 token 走通闭环

packages/llm_gateway/ww_llm_gateway/ledger.py

@@ -0,0 +1,39 @@
+"""用量记账落库（ARCH §4.8）。
+
+`LedgerSink` 为接口，便于测试注入内存替身；生产用 SQLAlchemy 实现写 usage_ledger。
+"""
+
+from __future__ import annotations
+
+from typing import Protocol
+
+from sqlalchemy.ext.asyncio import AsyncSession
+from ww_db.models import UsageLedger
+
+from .types import Scope, Usage
+
+
+class LedgerSink(Protocol):
+    async def record(self, scope: Scope, usage: Usage) -> None: ...
+
+
+class SqlAlchemyLedgerSink:
+    """把每次调用写入 usage_ledger（owner_id 取 scope.user_id，单用户 stub）。"""
+
+    def __init__(self, session: AsyncSession) -> None:
+        self._session = session
+
+    async def record(self, scope: Scope, usage: Usage) -> None:
+        row = UsageLedger(
+            owner_id=scope.user_id,
+            project_id=scope.project_id,
+            provider=usage.provider,
+            model=usage.model,
+            input_tokens=usage.input_tokens,
+            output_tokens=usage.output_tokens,
+            cache_read=usage.cache_read_tokens,
+            cost_minor=usage.cost_minor,
+            currency=usage.currency,
+        )
+        self._session.add(row)
+        await self._session.flush()