feat: extract_text.py – YouTube-URL → Textdatei in einem Schritt

- Kombiniert yt_untertitel.py und vtt2txt.py; die VTT bleibt temporär
- Standardsprache ist die des Original-Audios (original_language())
- -o Ausgabedatei, Standard output.txt
- Tests ohne Netz, README

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_yt_untertitel.py

@@ -63,6 +63,15 @@ def test_choose_track(info, lang, expected):
     assert yt.choose_track(info, lang) == expected
 
 
+@pytest.mark.parametrize("info, expected", [
+    (AUTO_ONLY, "de"),
+    ({"language": "fr", "automatic_captions": {}}, "fr"),
+    ({"subtitles": {"en": []}}, None),
+])
+def test_original_language(info, expected):
+    assert yt.original_language(info) == expected
+
+
 def test_available_tracks():
     info = {"subtitles": {"fr": []}, "automatic_captions": {"de-orig": [], "de": []}}
     assert yt.available_tracks(info) == (["fr"], ["de"])