feat(ai): Day 8 — 会话 CRUD API + chat_handler session 模式

- 新增 4 个会话端点: POST/GET /ai/chat/sessions, PUT rename, POST close
- ChatRequest 增加 session_id 字段（Optional，向后兼容）
- session_id 模式: DB 加载历史 + 持久化消息 + Tool 调用日志写入
- 无 session_id 时保持原有 history 数组模式不变
- 权限: ai.chat.session.list / ai.chat.session.manage

crates/erp-ai/src/module.rs

@@ -431,6 +431,22 @@ impl AiModule {
                 "/ai/chat",
                 axum::routing::post(crate::handler::chat_handler::chat),
             )
+            .route(
+                "/ai/chat/sessions",
+                axum::routing::post(crate::handler::chat_handler::create_session),
+            )
+            .route(
+                "/ai/chat/sessions",
+                axum::routing::get(crate::handler::chat_handler::list_sessions),
+            )
+            .route(
+                "/ai/chat/sessions/{session_id}/rename",
+                axum::routing::put(crate::handler::chat_handler::rename_session),
+            )
+            .route(
+                "/ai/chat/sessions/{session_id}/close",
+                axum::routing::post(crate::handler::chat_handler::close_session),
+            )
             .route(
                 "/ai/config",
                 axum::routing::get(crate::handler::config_handler::get_config),