feat: Phase 0 — monorepo 骨架 + 全表迁移 + FastAPI/Next 骨架 + CI

- uv(workspace) + pnpm monorepo；docker-compose(pg+api+web)
- SQLAlchemy 16 MVP 表 + Alembic 初版迁移(无漂移，users stub)
- FastAPI 骨架：统一错误信封(带 request_id) + structlog + /jobs/:id + OpenAPI
- Next.js 骨架：纸感主题 token + OpenAPI→TS 客户端代码生成(gen:api)
- CI(ruff/mypy/pytest + pg service + alembic 漂移校验)
- 四份设计规格(PRODUCT/UX/ARCHITECTURE/DEV_PLAN) + CLAUDE.md

packages/db/ww_db/session.py

@@ -0,0 +1,27 @@
+"""async engine / session（ARCH §2.3：同进程，async-first）。"""
+
+from __future__ import annotations
+
+from collections.abc import AsyncIterator
+from functools import lru_cache
+
+from sqlalchemy.ext.asyncio import (
+    AsyncSession,
+    async_sessionmaker,
+    create_async_engine,
+)
+from ww_config import get_settings
+
+
+@lru_cache
+def get_sessionmaker() -> async_sessionmaker[AsyncSession]:
+    settings = get_settings()
+    engine = create_async_engine(settings.database_url, pool_pre_ping=True)
+    return async_sessionmaker(engine, expire_on_commit=False)
+
+
+async def get_session() -> AsyncIterator[AsyncSession]:
+    """FastAPI 依赖：每请求一个 session。"""
+    sm = get_sessionmaker()
+    async with sm() as session:
+        yield session