feat(api): 拆分章节物化与 Story 后处理，并加固 Redis 锁与腾讯 ASR

回忆录 Story 流水线（同步）
- 同步路径仅写入 Story 与章节关联，改为 mark_chapter_dirty_sync，不再内联 compose
- 物化由 Celery recompose_chapter 异步完成；compose 不变量与异常时保留 dirty 的语义在 repo 中补充说明
- Evidence：大批次时降低 top_k；路由候选 story 携带 char_count/version_count；append 超长/版本过多时强制新开 story
- 叙事 prompt：relevant_chunks 去重，减少重复证据噪声
- 叙事回退与忠实度 gate：返回 fallback 类型并记录结构化日志（含耗时、JSON 有效性等）

Post-commit 与任务编排
- 新增 post_commit.enqueue_story_post_commit_effects：统一派发 generate_story_image（Redis 去重）、延迟 recompose_chapter、可选 memory compaction
- memoir_tasks / story_service / story_image_tasks 改为调用 post-commit 入口；主图回填后按关联章节重算并调度物化与 compacs（锁委托、Redis 单例、ASR to_thread）
- 更新 test_narrative_pipeline 以适配 _apply_narrative_fallbacks 返回值

api/app/tasks/chapter_compose_tasks.py

@@ -5,6 +5,11 @@ from datetime import datetime, timezone
 from celery import shared_task
 from sqlalchemy import select
 
+from app.core.chapter_pipeline_lock import (
+    acquire_chapter_pipeline_lock,
+    release_chapter_pipeline_lock,
+)
+from app.core.config import settings
 from app.core.db import get_sync_db
 from app.core.logging import get_logger
 from app.core.memory_compaction_schedule import schedule_memory_compaction_run
@@ -15,13 +20,85 @@ from app.features.story.models import Story
 logger = get_logger(__name__)
 
 
+@shared_task(bind=True, max_retries=3, default_retry_delay=30)
+def recompose_chapter(self, chapter_id: str) -> dict:
+    """
+    按章节物化 canonical_markdown：仅当 markdown_compose_dirty 为 True 时执行；
+    与 pipeline 共用章节级 Redis 锁，拿不到锁则跳过（依赖后续触发重试）。
+    """
+    lock_ttl = int(settings.chapter_pipeline_lock_ttl_seconds)
+    user_id: str | None = None
+    composed = False
+    with get_sync_db() as session:
+        chapter = session.get(Chapter, chapter_id)
+        if not chapter:
+            logger.info("recompose_chapter: chapter_id={} status=not_found", chapter_id)
+            return {"status": "not_found"}
+        if chapter.markdown_compose_dirty is not True:
+            logger.info(
+                "recompose_chapter: chapter_id={} status=skip_not_dirty",
+                chapter_id,
+            )
+            return {"status": "skip_not_dirty"}
+        uid = str(chapter.user_id)
+        stage = str(chapter.category)
+        lock_handle = acquire_chapter_pipeline_lock(uid, stage, ttl_seconds=lock_ttl)
+        if lock_handle is None:
+            logger.info(
+                "event=recompose_chapter status=skip_lock_contention "
+                "chapter_id={} user_id={} stage={}",
+                chapter_id,
+                uid,
+                stage,
+            )
+            return {"status": "skip_lock_contention"}
+        try:
+            composed = memoir_repo.compose_chapter_from_story_links_sync(
+                session, chapter_id
+            )
+            session.commit()
+            user_id = uid
+        except Exception as exc:
+            session.rollback()
+            logger.warning(
+                "recompose_chapter failed chapter_id={} err={}", chapter_id, exc
+            )
+            raise self.retry(exc=exc) from exc
+        finally:
+            release_chapter_pipeline_lock(lock_handle)
+
+    if user_id:
+        schedule_memory_compaction_run(
+            user_id,
+            {
+                "trigger_source": "chapter_recompose",
+                "trigger_time": datetime.now(timezone.utc).isoformat(),
+                "pipeline_run_id": str(self.request.id),
+                "recomposed_chapter_ids": [chapter_id],
+            },
+        )
+    logger.info(
+        "recompose_chapter: chapter_id={} status={}",
+        chapter_id,
+        "composed" if composed else "empty",
+    )
+    return {"status": "composed" if composed else "empty", "chapter_id": chapter_id}
+
+
 @shared_task(bind=True, max_retries=3, default_retry_delay=30)
 def recompose_chapters_for_story(self, story_id: str) -> dict:
+    """
+    按 story 找出 dirty 章节并物化。
+
+    .. deprecated::
+        请改用 `recompose_chapter`（按章聚合）+ `enqueue_story_post_commit_effects`；
+        保留兼容，待调用方全部迁移后删除。
+    """
     user_id: str | None = None
     try:
         with get_sync_db() as session:
             story = session.get(Story, story_id)
-            user_id = story.user_id if story else None
+            user_id = str(story.user_id) if story else None
             stmt = (
                 select(Chapter.id)
                 .join(