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/agents/memoir/__init__.py

@@ -2,6 +2,7 @@
 
 from app.agents.memoir.classification_agent import ClassificationAgent
 from app.agents.memoir.extraction_agent import ExtractionAgent, ExtractionResult
+from app.agents.memoir.fidelity_check_agent import FidelityCheckAgent
 from app.agents.memoir.narrative_agent import NarrativeAgent
 from app.agents.memoir.orchestrator import MemoirOrchestrator, PreparedMemoirBatches
 from app.agents.memoir.story_route_agent import (
@@ -24,4 +25,5 @@ __all__ = [
     "ExtractionResult",
     "ClassificationAgent",
     "NarrativeAgent",
+    "FidelityCheckAgent",
 ]