Add automated unit and mock integration test suite

- Configure conftest.py with pytest tmp_path DB isolation fixtures
- Add test_db.py for schema connection validations
- Add test_fetcher.py with mocked yt-dlp subprocess parser checks
- Add test_summarizer.py with mocked transcripts and Claude API requests
- Add test_main.py for FastAPI router endpoints and state updates

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

backend/tests/conftest.py

@@ -0,0 +1,44 @@
+import os
+import pytest
+import asyncio
+import aiosqlite
+import sys
+
+# Ensure backend folder is in path for imports
+backend_dir = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
+if backend_dir not in sys.path:
+    sys.path.insert(0, backend_dir)
+
+import db
+
+@pytest.fixture(scope="session")
+def event_loop():
+    try:
+        loop = asyncio.get_running_loop()
+    except RuntimeError:
+        loop = asyncio.new_event_loop()
+    yield loop
+    loop.close()
+
+@pytest.fixture(autouse=True)
+def mock_db_path(tmp_path, monkeypatch):
+    # Ensure all modules (db, fetcher, summarizer, main) point to the temp test db
+    test_db_file = os.path.join(tmp_path, "test_t_youtube.db")
+    monkeypatch.setattr(db, "DB_PATH", test_db_file)
+
+    # Also monkeypatch in other modules that import DB_PATH directly
+    import fetcher
+    import summarizer
+    monkeypatch.setattr(fetcher, "DB_PATH", test_db_file)
+    monkeypatch.setattr(summarizer, "DB_PATH", test_db_file)
+
+    return test_db_file
+
+@pytest.fixture
+async def test_db(mock_db_path):
+    # Initialize the tables
+    await db.init_db()
+
+    async with aiosqlite.connect(mock_db_path) as conn:
+        conn.row_factory = aiosqlite.Row
+        yield conn