重构回忆录为 story-first / markdown-first 架构并整合图片意图与前端 UI 修复

本次 squash merge 将 codex-story-first-image-intent 的整体改动合入 development，核心内容包括：

1. 后端数据与迁移：新增 stories、story_versions、story_image_intents、chapter_cover_intents、assets 等模型与 Alembic 迁移，建立 story-first、markdown-first、asset-first 的主数据链路。

2. 生成与任务链：引入 StoryBuilderOrchestrator、ChapterComposerOrchestrator、story_image_tasks、chapter_cover_tasks，图片生成从正文占位符改为结构化 intent -> asset -> markdown 回填。

3. 并发与一致性：为 story/chapter intent 增加 claim_token、claimed_at、attempt_count，采用数据库原子 claim 为主、Redis 锁为辅，避免重复生成、锁误删和 processing 卡死。

4. Memoir 读写路径：章节 canonical_markdown 成为正文真源，列表/详情接口补齐 markdown、cover_asset、word_count 等字段，PDF 与 asset 解析链路同步升级。

5. Memory / Retrieval：扩展 transcript ingest、chunking、evidence 检索与 story 聚合基础设施，为后续 story-first RAG 与多 agent 编排提供底座。

6. App 端体验：章节页继续走 MarkdownRenderer 阅读链，同时吸收 fix3-19 的跨平台 UI glitch 修复；更新对话页、首页、文案资源与章节列表映射逻辑。

7. 测试与文档：补充 asset resolver、story image task、章节封面派发、markdown 映射等回归测试，并加入图片占位符退役设计文档。

api/app/features/story/service.py

@@ -0,0 +1,228 @@
+"""
+StoryService — Story 层业务逻辑。
+
+- 创建 story、版本、evidence 关联
+- 不直接依赖 agent，由 orchestrator 调用
+- story 正文生成后提取 primary image intent 并落库
+"""
+
+from datetime import datetime, timezone
+
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.core.logging import get_logger
+from app.features.story.image_intent_extractor import extract_primary_image_intent
+from app.features.story.repo import (
+    count_story_versions,
+    create_story,
+    create_story_evidence_link,
+    create_story_image_intent,
+    create_story_version,
+    delete_story_image_intents_by_story,
+    get_stories_for_user,
+    get_story_by_id,
+    get_story_image_intent_by_story,
+)
+
+logger = get_logger(__name__)
+
+
+async def _extract_and_store_image_intent(
+    db,
+    *,
+    story,
+    version,
+    markdown: str,
+) -> None:
+    """
+    从 markdown 提取 primary intent。
+    仅移除 pending/failed，避免删掉正在 processing 的旧任务行；同版本则原地更新行以幂等。
+    """
+    await delete_story_image_intents_by_story(
+        db, story.id, statuses=["pending", "failed"]
+    )
+    result = extract_primary_image_intent(
+        markdown,
+        title=story.title or "",
+        stage=story.stage,
+        summary=story.summary,
+        people_refs=story.people_refs or [],
+        place_refs=story.place_refs or [],
+        time_start=story.time_start,
+        time_end=story.time_end,
+    )
+    existing = await get_story_image_intent_by_story(db, story.id, role="primary")
+    now = datetime.now(timezone.utc)
+
+    if existing and existing.story_version_id == version.id:
+        st = (existing.status or "").strip()
+        if st in ("processing", "completed"):
+            return
+        existing.caption = result.caption
+        existing.prompt_brief = result.prompt_brief
+        existing.source_span = result.source_span
+        existing.style_profile = result.style_profile
+        existing.status = "pending"
+        existing.error = None
+        existing.asset_id = None
+        existing.updated_at = now
+        return
+
+    if existing and existing.story_version_id != version.id:
+        # 复用同一主键行，避免删行导致进行中的 Celery 任务找不到 intent
+        existing.story_version_id = version.id
+        existing.caption = result.caption
+        existing.prompt_brief = result.prompt_brief
+        existing.source_span = result.source_span
+        existing.style_profile = result.style_profile
+        existing.status = "pending"
+        existing.error = None
+        existing.asset_id = None
+        existing.updated_at = now
+        return
+
+    await create_story_image_intent(
+        db,
+        story_id=story.id,
+        story_version_id=version.id,
+        caption=result.caption,
+        prompt_brief=result.prompt_brief,
+        source_span=result.source_span,
+        style_profile=result.style_profile,
+    )
+
+
+class StoryService:
+    def __init__(self, db: AsyncSession):
+        self._db = db
+
+    async def create_story(
+        self,
+        user_id: str,
+        title: str,
+        *,
+        stage: str | None = None,
+        story_type: str | None = None,
+        summary: str | None = None,
+        canonical_markdown: str | None = None,
+    ) -> str:
+        """Create story, commit, return story_id."""
+        story = await create_story(
+            self._db,
+            user_id=user_id,
+            title=title,
+            stage=stage,
+            story_type=story_type,
+            summary=summary,
+            canonical_markdown=canonical_markdown or "",
+        )
+        await self._db.flush()
+        if canonical_markdown:
+            version = await create_story_version(
+                self._db,
+                story_id=story.id,
+                version_no=1,
+                markdown_snapshot=canonical_markdown,
+                actor_type="ai",
+                source_type="generate",
+            )
+            await self._db.flush()
+            story.current_version_id = version.id
+            await _extract_and_store_image_intent(
+                self._db,
+                story=story,
+                version=version,
+                markdown=canonical_markdown,
+            )
+        await self._db.commit()
+        if canonical_markdown:
+            from app.tasks.story_image_tasks import generate_story_image
+
+            try:
+                generate_story_image.delay(story.id)
+            except Exception as exc:
+                logger.warning("派发 generate_story_image 失败: %s", exc)
+        return story.id
+
+    async def append_version(
+        self,
+        story_id: str,
+        markdown_snapshot: str,
+        *,
+        actor_type: str = "ai",
+        source_type: str = "generate",
+        change_summary: str | None = None,
+        prompt_meta: dict | None = None,
+    ) -> str:
+        """Append new version, update canonical_markdown, return version_id."""
+        story = await get_story_by_id(self._db, story_id)
+        if not story:
+            raise ValueError(f"Story {story_id} not found")
+        parent_id = story.current_version_id
+        version_no = (await count_story_versions(self._db, story_id)) + 1
+        version = await create_story_version(
+            self._db,
+            story_id=story_id,
+            version_no=version_no,
+            markdown_snapshot=markdown_snapshot,
+            actor_type=actor_type,
+            source_type=source_type,
+            parent_version_id=parent_id,
+            prompt_meta=prompt_meta,
+        )
+        version.change_summary = change_summary
+        story.current_version_id = version.id
+        story.canonical_markdown = markdown_snapshot
+        await _extract_and_store_image_intent(
+            self._db,
+            story=story,
+            version=version,
+            markdown=markdown_snapshot,
+        )
+        await self._db.commit()
+        from app.tasks.story_image_tasks import generate_story_image
+
+        try:
+            generate_story_image.delay(story_id)
+        except Exception as exc:
+            logger.warning("派发 generate_story_image 失败: %s", exc)
+        return version.id
+
+    async def link_evidence(
+        self,
+        story_id: str,
+        evidence_type: str,
+        evidence_id: str,
+        *,
+        role: str = "primary",
+        weight: float | None = None,
+    ) -> None:
+        """Add evidence link. Caller must ensure story exists."""
+        await create_story_evidence_link(
+            self._db,
+            story_id=story_id,
+            evidence_type=evidence_type,
+            evidence_id=evidence_id,
+            role=role,
+            weight=weight,
+        )
+        await self._db.commit()
+
+    async def get_stories(
+        self, user_id: str, *, status: str | None = "active"
+    ) -> list[dict]:
+        """List stories for user."""
+        stories = await get_stories_for_user(self._db, user_id, status=status)
+        return [
+            {
+                "id": s.id,
+                "title": s.title,
+                "stage": s.stage,
+                "story_type": s.story_type,
+                "summary": s.summary,
+                "canonical_markdown": s.canonical_markdown,
+                "status": s.status,
+                "created_at": s.created_at.isoformat() if s.created_at else None,
+            }
+            for s in stories
+        ]