fix: isolate tests from real output/ dir via conftest autouse fixture

Tests imported the global job_queue (bound to real OUTPUT_DIR), creating
8 junk job-* dirs per pytest run (test_app x3, test_download_compress x4,
test_preview x1). Fixture redirects _output_dir to tmp_path and clears
the job registry before each test.

tests/conftest.py

@@ -0,0 +1,19 @@
+"""Global test isolation — keep tests out of the real output/ directory.
+
+Tests import the global job_queue from app.py, which is bound to the real
+OUTPUT_DIR. Without this, every pytest run leaves 8 junk job-* directories
+in output/ (test_app × 3, test_download_compress × 4, test_preview × 1).
+"""
+
+import pytest
+
+
+@pytest.fixture(autouse=True)
+def _isolate_job_queue(tmp_path):
+    import app as app_module
+
+    q = app_module.job_queue
+    with q._lock:
+        q._output_dir = tmp_path / "output"
+        q._jobs.clear()
+    yield