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/components/Toast.tsx

@@ -0,0 +1,73 @@
+"use client";
+
+import {
+  createContext,
+  useCallback,
+  useContext,
+  useMemo,
+  useState,
+  type ReactNode,
+} from "react";
+
+type ToastKind = "info" | "error" | "success";
+
+interface ToastItem {
+  id: number;
+  message: string;
+  kind: ToastKind;
+}
+
+type ShowToast = (message: string, kind?: ToastKind) => void;
+
+const ToastContext = createContext<ShowToast | null>(null);
+
+const TOAST_TTL_MS = 4000;
+
+export function ToastProvider({ children }: { children: ReactNode }) {
+  const [items, setItems] = useState<ToastItem[]>([]);
+
+  const show = useCallback<ShowToast>((message, kind = "info") => {
+    const id = Date.now() + Math.random();
+    setItems((prev) => [...prev, { id, message, kind }]);
+    setTimeout(() => {
+      setItems((prev) => prev.filter((t) => t.id !== id));
+    }, TOAST_TTL_MS);
+  }, []);
+
+  const value = useMemo(() => show, [show]);
+
+  return (
+    <ToastContext.Provider value={value}>
+      {children}
+      <div
+        className="pointer-events-none fixed bottom-6 right-6 z-50 flex flex-col gap-2"
+        aria-live="polite"
+        role="status"
+      >
+        {items.map((t) => (
+          <div
+            key={t.id}
+            className={`pointer-events-auto rounded border px-4 py-2 text-sm shadow-paper ${
+              t.kind === "error"
+                ? "border-conflict bg-panel text-conflict"
+                : t.kind === "success"
+                  ? "border-pass bg-panel text-pass"
+                  : "border-line bg-panel text-ink"
+            }`}
+          >
+            {t.message}
+          </div>
+        ))}
+      </div>
+    </ToastContext.Provider>
+  );
+}
+
+export function useToast(): ShowToast {
+  const ctx = useContext(ToastContext);
+  if (!ctx) {
+    // 容错：未挂 Provider 时退化为 no-op（不应在生产路径发生）。
+    return () => undefined;
+  }
+  return ctx;
+}