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

apps/web/lib/api/client.test.ts

@@ -0,0 +1,8 @@
+import { describe, expect, it } from "vitest";
+
+describe("api base url", () => {
+  it("falls back to localhost when env unset", () => {
+    const base = process.env.NEXT_PUBLIC_API_BASE ?? "http://localhost:8000";
+    expect(base).toMatch(/^https?:\/\//);
+  });
+});