Detect a dead local pipeline worker after API restart, and flush SSE.

A docker rebuild left HubAudio status=running with no process, so the UI sat on a silent event stream. Persist the worker pid and disable proxy buffering on the SSE response.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/test_job_runner.py

@@ -0,0 +1,11 @@
+"""Local worker pid file so a restarted API can see a dead subprocess."""
+
+from backend.services import job_runner
+
+
+def test_local_state_failed_when_pid_file_is_stale(tmp_path, monkeypatch):
+    monkeypatch.setattr(job_runner.settings, "data_dir", tmp_path)
+    pid_path = tmp_path / "workers" / "proj.pid"
+    pid_path.parent.mkdir(parents=True)
+    pid_path.write_text("99999999")
+    assert job_runner.get_execution_state("local/projects/proj") == "failed"