Fix youtube-transcript-api call structure and make test assertions environment-robust

- Instantiate YouTubeTranscriptApi class and call list() as required by v1.2.4
- Decorate async database fixture with pytest_asyncio.fixture
- Replace hardcoded base URL string assertion in test_summarizer.py with dynamized summarizer.BASE_URL value to support proxy profiles

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/tests/conftest.py

@@ -1,5 +1,6 @@
 import os
 import pytest
+import pytest_asyncio
 import asyncio
 import aiosqlite
 import sys
@@ -34,7 +35,7 @@ def mock_db_path(tmp_path, monkeypatch):
 
     return test_db_file
 
-@pytest.fixture
+@pytest_asyncio.fixture
 async def test_db(mock_db_path):
     # Initialize the tables
     await db.init_db()