chore: Cargo warnings 清零 — 39→0 (仅剩 sqlx-postgres 外部依赖警告)

- runtime: 移除未使用的 SessionId/Datelike import，修复 unused variable
- intelligence: 模块级 #![allow(dead_code)] 抑制 Hermes 预留代码警告
- mcp.rs/persist.rs/nl_schedule.rs: 标注 #[allow(dead_code)] 保留接口

desktop/src-tauri/src/classroom_commands/persist.rs

@@ -101,6 +101,7 @@ impl ClassroomPersistence {
     }
 
     /// Delete a classroom and its chat history.
+    #[allow(dead_code)]
     pub async fn delete_classroom(&self, classroom_id: &str) -> Result<(), String> {
         let mut conn = self.conn.lock().await;
         sqlx::query("DELETE FROM classrooms WHERE id = ?")