fix(import): never treat the download dir itself as one audiobook

tests/test_import_flow.py

@@ -965,3 +965,32 @@ def test_scan_and_import_survive_a_name_that_is_not_utf8(client):
         "Die drei ??? und die Fußball-Falle - Part 02.mp3",
     ]
     assert not folder.exists()
+
+
+def test_loose_files_in_the_download_root_do_not_swallow_everything(client):
+    """Two loose audio files directly in the download dir used to make the whole
+    root look like one audiobook - and its early return hid every subfolder."""
+    (client.downloads / "Teil 01.mp3").write_bytes(b"")
+    (client.downloads / "Teil 02.mp3").write_bytes(b"")
+    for folge in ("Folge 1", "Folge 2"):
+        d = client.downloads / "Serie" / folge
+        d.mkdir(parents=True)
+        for i in (1, 2):
+            (d / f"{i:02d}.mp3").write_bytes(b"")
+
+    items = client.get("/api/import/scan").json()["items"]
+    assert sorted(i["name"] for i in items) == ["Folge 1", "Folge 2", "Teil 01", "Teil 02"]
+    # the folders stay grouped, the loose files are listed on their own
+    by_name = {i["name"]: i for i in items}
+    assert by_name["Folge 1"]["is_dir"] and len(by_name["Folge 1"]["files"]) == 2
+    assert not by_name["Teil 01"]["is_dir"] and len(by_name["Teil 01"]["files"]) == 1
+
+
+def test_a_single_folder_with_many_files_is_still_one_audiobook(client):
+    """The fix must not break the normal case one level down."""
+    d = client.downloads / "Ein Hoerbuch"
+    d.mkdir()
+    for i in (1, 2, 3):
+        (d / f"{i:02d}.mp3").write_bytes(b"")
+    items = client.get("/api/import/scan").json()["items"]
+    assert len(items) == 1 and items[0]["is_dir"] and len(items[0]["files"]) == 3