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

apps/web/lib/stream/useDraftStream.ts

@@ -0,0 +1,122 @@
+"use client";
+
+import { useCallback, useReducer, useRef } from "react";
+
+import { API_BASE_PUBLIC } from "@/lib/api/config";
+import {
+  SseFrameBuffer,
+  initialStreamState,
+  reduceStream,
+  type StreamState,
+} from "./sse";
+
+type Action =
+  | { type: "start" }
+  | { type: "events"; events: ReturnType<SseFrameBuffer["push"]> }
+  | { type: "abort" }
+  | { type: "fail"; code: string; message: string }
+  | { type: "reset"; text: string };
+
+function reducer(state: StreamState, action: Action): StreamState {
+  switch (action.type) {
+    case "start":
+      return { phase: "streaming", text: "", error: null };
+    case "events":
+      return action.events.reduce(reduceStream, state);
+    case "abort":
+      return { ...state, phase: "aborted" };
+    case "fail":
+      return {
+        ...state,
+        phase: "error",
+        error: { code: action.code, message: action.message },
+      };
+    case "reset":
+      return { ...initialStreamState, text: action.text };
+    default:
+      return state;
+  }
+}
+
+export interface UseDraftStream {
+  state: StreamState;
+  isStreaming: boolean;
+  start: (projectId: string, chapterNo: number) => Promise<void>;
+  stop: () => void;
+  reset: (text: string) => void;
+}
+
+// 消费 POST .../draft 的 SSE 流。用 fetch+ReadableStream（EventSource 不支持 POST）。
+// "停" = abort 连接（已生成部分保留在 state.text，由调用方落入草稿）。
+export function useDraftStream(): UseDraftStream {
+  const [state, dispatch] = useReducer(reducer, initialStreamState);
+  const controllerRef = useRef<AbortController | null>(null);
+
+  const stop = useCallback(() => {
+    controllerRef.current?.abort();
+    controllerRef.current = null;
+    dispatch({ type: "abort" });
+  }, []);
+
+  const reset = useCallback((text: string) => {
+    dispatch({ type: "reset", text });
+  }, []);
+
+  const start = useCallback(async (projectId: string, chapterNo: number) => {
+    const controller = new AbortController();
+    controllerRef.current = controller;
+    dispatch({ type: "start" });
+    try {
+      const res = await fetch(
+        `${API_BASE_PUBLIC}/projects/${projectId}/chapters/${chapterNo}/draft`,
+        {
+          method: "POST",
+          headers: { Accept: "text/event-stream" },
+          signal: controller.signal,
+        },
+      );
+      if (!res.ok || !res.body) {
+        // 流前错误（如无凭据 → 503 LLM_UNAVAILABLE，JSON 信封而非帧）。
+        let code = "STREAM_FAILED";
+        let message = `写章请求失败（${res.status}）`;
+        try {
+          const body = (await res.json()) as {
+            error?: { code?: string; message?: string };
+          };
+          if (body.error?.code) code = body.error.code;
+          if (body.error?.message) message = body.error.message;
+        } catch {
+          // 非 JSON 信封，沿用默认文案。
+        }
+        dispatch({ type: "fail", code, message });
+        return;
+      }
+      const reader = res.body.getReader();
+      const decoder = new TextDecoder();
+      const buffer = new SseFrameBuffer();
+      for (;;) {
+        const { value, done } = await reader.read();
+        if (done) break;
+        const events = buffer.push(decoder.decode(value, { stream: true }));
+        if (events.length > 0) dispatch({ type: "events", events });
+      }
+    } catch (err: unknown) {
+      if (err instanceof DOMException && err.name === "AbortError") {
+        // 用户主动停止：state 已置 aborted。
+        return;
+      }
+      const message = err instanceof Error ? err.message : "未知网络错误";
+      dispatch({ type: "fail", code: "NETWORK", message });
+    } finally {
+      controllerRef.current = null;
+    }
+  }, []);
+
+  return {
+    state,
+    isStreaming: state.phase === "streaming",
+    start,
+    stop,
+    reset,
+  };
+}