fix(runner): use python -u for unbuffered stdout streaming

Python defaults to 4KB block buffering when stdout is a pipe (not a
TTY). Docker attach uses pipes, so print() output accumulated in the
buffer and only flushed when the process exited — the 3-second
sleep(1) loop appeared as a single dump at the end.

python -u (equivalent to PYTHONUNBUFFERED=1) forces line-level
flushing, so each print() is immediately written to the pipe and
picked up by the concurrent log reader → SSE → xterm.js.

Node/Go/Rust already line-buffer to pipes, so only Python needed the
flag. stdout/stderr separation is preserved (no TTY merge).

src/api/code_runner/languages.rs

@@ -33,7 +33,11 @@ pub static LANGUAGES: LazyLock<HashMap<String, LanguageDef>> = LazyLock::new(||
         "python".to_string(),
         LanguageDef {
             image: "yggdrasil-runner-python:latest".to_string(),
-            run_cmd: "python /code/main.py".to_string(),
+            // -u (unbuffered)：强制 stdout/stderr 行刷新。
+            // 容器 attach 用 pipe（非 TTY），Python 默认对 pipe 做块缓冲（4KB），
+            // 导致流式输出失效——print 的内容攒在缓冲区，进程退出才一次性刷出。
+            // -u 等价于 PYTHONUNBUFFERED=1，让每行 print 立即写出。
+            run_cmd: "python -u /code/main.py".to_string(),
             extension: "py".to_string(),
             default_limits: ResourceLimits {
                 cpu_cores: 1.0,