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/base.py

@@ -0,0 +1,29 @@
+"""SQLAlchemy 2.0 声明式基类 + 通用 mixin。"""
+
+from __future__ import annotations
+
+import uuid
+from datetime import datetime
+
+from sqlalchemy import Uuid, func, text
+from sqlalchemy.orm import DeclarativeBase, Mapped, mapped_column
+
+
+class Base(DeclarativeBase):
+    pass
+
+
+class UuidPk:
+    id: Mapped[uuid.UUID] = mapped_column(
+        Uuid, primary_key=True, server_default=text("gen_random_uuid()")
+    )
+
+
+class CreatedAt:
+    created_at: Mapped[datetime] = mapped_column(server_default=func.now(), nullable=False)
+
+
+class TimestampedMixin(CreatedAt):
+    updated_at: Mapped[datetime] = mapped_column(
+        server_default=func.now(), onupdate=func.now(), nullable=False
+    )