feat(ai): Day 7 — 会话持久化 Entity + Service

- 新增 3 个 SeaORM Entity: ai_chat_session / ai_chat_message / ai_tool_call_log
- ChatSessionService: create / list / get / close / rename
- ChatMessageService: save_message / list_messages / save_tool_call_log
- 参数封装为 SaveMessageParams / SaveToolCallLogParams 避免 clippy too_many_arguments
- AiState 注册 chat_session + chat_message 服务
- erp-server main.rs 初始化注入

crates/erp-ai/src/service/mod.rs

@@ -2,6 +2,8 @@ pub mod analysis;
 pub mod analysis_queue;
 pub mod auto_analysis;
 pub mod cache;
+pub mod chat_message;
+pub mod chat_session;
 pub mod comparison;
 pub mod cost;
 pub mod dialysis_risk_scorer;