feat(api+app): 对话阶段化、回忆录流水线与客户端会话体验

- DB: segments 用户输入文本（Alembic 0002）
- Chat: 阶段检测/阶段提示/回复限制，编排与访谈/画像 prompts 调整
- Memoir: 忠实度检查 agent，叙事与分类等链路更新
- Core: agent 日志、Alembic 启动、LangChain/日志/配置等
- Story: time_hints；Memory 检索与相关测试
- Expo: 助手头像、会话页与消息拆分、实时会话与文案/i18n
- Docs/scripts/tests: 迁移脚本、LLM JSON/记忆检索文档、新增单测

api/app/features/story/sync_write.py

@@ -19,6 +19,7 @@ from app.features.memoir.models import ChapterStoryLink
 from app.features.memoir import repo as memoir_repo
 from app.features.story.image_intent_extractor import extract_primary_image_intent
 from app.features.story.models import Story, StoryImageIntent, StoryVersion
+from app.features.story.time_hints import apply_infer_story_time_start_to_model
 
 logger = get_logger(__name__)
 
@@ -137,6 +138,7 @@ def create_story_with_version_sync(
     session.add(version)
     session.flush()
     story.current_version_id = vid
+    apply_infer_story_time_start_to_model(story)
     if md.strip():
         _extract_and_store_image_intent_sync(
             session, story=story, version=version, markdown=md
@@ -173,6 +175,7 @@ def append_story_version_sync(
     session.flush()
     story.current_version_id = vid
     story.canonical_markdown = md
+    apply_infer_story_time_start_to_model(story)
     _extract_and_store_image_intent_sync(
         session, story=story, version=version, markdown=md
     )