feat: storage management — disk usage, cleanup API, scheduled cleanup task

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/tests/test_storage.py

@@ -0,0 +1,26 @@
+import os
+import pytest
+from unittest.mock import patch
+
+from app.services.storage import get_disk_usage, _dir_size
+
+
+def test_dir_size_empty(tmp_path):
+    assert _dir_size(str(tmp_path)) == 0
+
+
+def test_dir_size_with_files(tmp_path):
+    f = tmp_path / "test.bin"
+    f.write_bytes(b"\x00" * 1024)
+    assert _dir_size(str(tmp_path)) == 1024
+
+
+def test_dir_size_nonexistent():
+    assert _dir_size("/nonexistent/path/12345") == 0
+
+
+def test_get_disk_usage():
+    with patch("app.services.storage._dir_size", return_value=1024 * 1024 * 512):
+        result = get_disk_usage()
+    assert result["total_bytes"] == 1024 * 1024 * 512 * 2
+    assert "total_gb" in result