feat(backend): update database models and add agent_conversation

- base.py: update database base configuration
- models/__init__.py: update models exports
- models/system_setting.py: update system setting model
- models/agent_conversation.py: add new agent conversation model

server/src/app/db/base.py

@@ -1,4 +1,5 @@
 from app.db.base_class import Base
+from app.models.agent_conversation import AgentConversation, AgentConversationMessage
 from app.models.agent_asset import AgentAsset, AgentAssetReview, AgentAssetVersion
 from app.models.agent_run import AgentRun, AgentToolCall, SemanticParseLog
 from app.models.approval import ApprovalRecord
@@ -22,6 +23,8 @@ __all__ = [
     "Base",
     "AccountsPayableRecord",
     "AccountsReceivableRecord",
+    "AgentConversation",
+    "AgentConversationMessage",
     "AgentAsset",
     "AgentAssetReview",
     "AgentAssetVersion",