feat(im): mirror WeChat bot chats into AI chat as WeXin Bot conversation; add conversations.source; migrate history; remove redundant history page

backend/python/app/models.py

@@ -85,6 +85,7 @@ class Conversation(Base):
     pinned = Column(Boolean, default=False)
     knowledge_graph_count = Column(Integer, default=0)
     knowledge_graph_base_id = Column(String(64), default=None, comment="已生成的知识图谱知识库ID")
+    source = Column(String(32), default=None, comment="对话来源，如 wechat_bot")
     created_at = Column(DateTime, default=datetime.now)
     updated_at = Column(DateTime, default=datetime.now, onupdate=datetime.now)