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-server/src/main.rs

@@ -615,6 +615,12 @@ async fn main() -> anyhow::Result<()> {
                     erp_ai::service::embedding::EmbeddingService::from_settings(&db).await,
                 ),
             )),
+            chat_session: std::sync::Arc::new(
+                erp_ai::service::chat_session::ChatSessionService::new(db.clone()),
+            ),
+            chat_message: std::sync::Arc::new(
+                erp_ai::service::chat_message::ChatMessageService::new(db.clone()),
+            ),
         }
     };