stream_command used a blocking proc.stdout.readline() that never woke
when the underlying journalctl was silent, so Flask never delivered
GeneratorExit on client disconnect — the worker thread and the journalctl
child both leaked permanently and pinned the gunicorn thread pool.
Switch to a select-based read loop with a 15s heartbeat tick (yielded as
""), and translate the tick to an SSE keepalive comment in the log route.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
