feat(story): auto-generate chapter titles for continuations

Phase 5 PR-E. Now that stories can grow chapter-by-chapter (PR-D Lite),
each new chapter benefits from its own title for navigation.

Changes:
- /api/stories/[id]/continue now calls gpt-4o-mini after writing the
  body to produce a 4-10 character poetic chapter title; saves it
  alongside the chapter and surfaces it in the SSE complete event
- The continuation dialog already shows the message, so users see
  "第 2 章「夜半潮声」已添加" in the success toast
- /api/stories no longer copies the story title onto the first chapter
  — that was creating a duplicate h2 in the reader
- StoryReader skips the chapter h2 entirely for single-chapter stories
  (the page h1 already serves as the title) and defensively hides
  chapter titles that equal the story title for legacy data

The chapter title prompt is intentionally conservative (no spoilers,
poetic but short, no "第 X 章" prefix) and falls back to null on any
failure; the reader handles untitled chapters as "第 N 章".

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: ChanMeng666

src/app/api/stories/[id]/continue/route.ts

@@ -20,6 +20,41 @@ const CONTINUE_PROMPT = `你是一位顶尖的同人文作者，正在为一篇
 
 输出要求：直接输出本章正文，不要加章节标题、不要加作者注、不要加任何元信息。`;
 
+const CHAPTER_TITLE_PROMPT = `请为以下同人文章节拟一个简洁的中文章节标题。
+
+要求：
+1. 4~10 字
+2. 有诗意，能引发读者兴趣
+3. 不要剧透关键转折
+4. 不要带「第 X 章」字样，不要引号
+5. 直接输出标题，不要任何前缀或额外文字
+
+只输出标题本身。`;
+
+async function generateChapterTitle(content: string): Promise<string | null> {
+  try {
+    const model = new ChatOpenAI({
+      temperature: 0.6,
+      model: "gpt-4o-mini",
+      maxTokens: 30,
+    });
+    const res = await model.invoke([
+      new SystemMessage(CHAPTER_TITLE_PROMPT),
+      new HumanMessage(content.slice(0, 3000)),
+    ]);
+    const raw = (typeof res.content === "string" ? res.content : "").trim();
+    const cleaned = raw
+      .replace(/^["「『《]|["」』》]$/g, "")
+      .replace(/^第.{1,4}章[：:]?\s*/, "")
+      .trim();
+    if (cleaned.length < 2 || cleaned.length > 20) return null;
+    return cleaned;
+  } catch (e) {
+    console.warn("[continue] chapter title generation failed:", e);
+    return null;
+  }
+}
+
 interface ContinueBody {
   direction: string;
 }
@@ -158,14 +193,17 @@ ${direction}
           return;
         }
 
+        send({ stage: "titling", message: "正在为本章拟标题…" });
+        const chapterTitle = await generateChapterTitle(content);
+
         send({ stage: "saving", message: "正在保存章节…" });
 
         const wordCount = countWords(content);
 
         const newChapter = await prisma.chapter.create({
           data: {
             storyId,
-            title: null,
+            title: chapterTitle,
             content,
             chapterNumber: nextChapterNumber,
             wordCount,
@@ -179,9 +217,12 @@ ${direction}
 
         send({
           stage: "complete",
-          message: `第 ${nextChapterNumber} 章已添加（${wordCount.toLocaleString()} 字）`,
+          message: chapterTitle
+            ? `第 ${nextChapterNumber} 章「${chapterTitle}」已添加（${wordCount.toLocaleString()} 字）`
+            : `第 ${nextChapterNumber} 章已添加（${wordCount.toLocaleString()} 字）`,
           chapterId: newChapter.id,
           chapterNumber: nextChapterNumber,
+          chapterTitle,
           wordCount,
         });
       } catch (err) {

src/app/api/stories/route.ts

@@ -54,7 +54,12 @@ export async function POST(req: NextRequest) {
         authorId: dbUser.id,
         chapters: {
           create: {
-            title: result.title,
+            // Don't duplicate the story title onto its only chapter — when
+            // a second chapter is added later via /api/stories/[id]/continue
+            // it gets an auto-generated chapter title, and the reader UI
+            // relies on chapter.title being null/distinct to avoid showing
+            // a redundant h2.
+            title: null,
             content: result.body,
             chapterNumber: 1,
             wordCount: result.wordCount,

src/components/story/StoryReader.tsx

@@ -165,11 +165,15 @@ export function StoryReader({
         <div className="space-y-12">
           {story.chapters.map((chapter, idx) => (
             <section key={chapter.id} className="space-y-4">
-              {(chapter.title || chapterCount > 1) && (
+              {/* Skip chapter h2 entirely for single-chapter stories — the
+                  page h1 (story title) already serves as the heading and a
+                  second one looks duplicated. */}
+              {chapterCount > 1 && (
                 <h2 className="font-display text-xl md:text-2xl font-semibold text-foreground">
-                  {chapterCount > 1 ? `第 ${chapter.chapterNumber} 章` : null}
-                  {chapterCount > 1 && chapter.title ? "：" : ""}
-                  {chapter.title}
+                  第 {chapter.chapterNumber} 章
+                  {chapter.title && chapter.title !== story.title
+                    ? `：${chapter.title}`
+                    : ""}
                 </h2>
               )}
               <div className="font-prose text-foreground/90 leading-8 text-base md:text-lg whitespace-pre-wrap">