feat: migrate persistence from JSON to SQLModel (Phase 1)

- Integrated SQLModel with SQLite for robust data persistence
- Refactored UserManager and WatchlistManager to use SQL queries
- Migrated models to SQLModel with relationships and primary keys
- Updated test suite with in-memory database isolation
- Removed deprecated JSON storage files

tests/conftest.py

@@ -11,12 +11,66 @@ from unittest.mock import Mock, AsyncMock, patch
 import sys
 import os
 
+import sys
+import os
+
 # Ensure the project root is in the Python path
 sys.path.insert(0, os.path.abspath(os.path.join(os.path.dirname(__file__), '..')))
 
+# FORCE DATABASE_URL to in-memory for ALL tests before ANY app imports
+os.environ["DATABASE_URL"] = "sqlite://"
+
 from app.models import DownloadTask, DownloadStatus, DownloadRequest, HostType
 from app.favorites import FavoritesManager
 from app.download_manager import DownloadManager
+from sqlmodel import SQLModel, create_engine, Session
+
+
+@pytest.fixture(scope="session", autouse=True)
+def init_db():
+    """Initialize the in-memory database once for the test session"""
+    from app.database import engine
+    SQLModel.metadata.create_all(engine)
+    return engine
+
+
+@pytest.fixture(name="engine")
+def engine_fixture():
+    """Returns the global test engine"""
+    from app.database import engine
+    return engine
+
+
+@pytest.fixture(name="session")
+def session_fixture(engine):
+    """Create a temporary database session for testing"""
+    # Clear and recreate tables for each test to ensure isolation
+    SQLModel.metadata.drop_all(engine)
+    SQLModel.metadata.create_all(engine)
+    with Session(engine) as session:
+        yield session
+
+
+@pytest.fixture(autouse=True)
+def mock_db(engine):
+    """Ensure each test starts with fresh tables"""
+    SQLModel.metadata.drop_all(engine)
+    SQLModel.metadata.create_all(engine)
+    yield engine
+
+
+@pytest.fixture
+def user_manager():
+    """Create a UserManager instance"""
+    from app.auth import UserManager
+    return UserManager()
+
+
+@pytest.fixture
+def watchlist_manager():
+    """Create a WatchlistManager instance"""
+    from app.watchlist import WatchlistManager
+    return WatchlistManager()
 
 
 def pytest_configure(config):