feat: implement Fitbit OAuth, Garmin MFA, and optimize segment discovery

- Add Fitbit authentication flow (save credentials, OAuth callback handling)
- Implement Garmin MFA support with successful session/cookie handling
- Optimize segment discovery with new sampling and activity query services
- Refactor database session management in discovery API for better testability
- Enhance activity data parsing for charts and analysis
- Update tests to use testcontainers and proper dependency injection
- Clean up repository by ignoring and removing tracked transient files (.pyc, .db)

FitnessSync/backend/tests/api/test_discovery.py

@@ -3,9 +3,8 @@ from main import app
 from unittest.mock import MagicMock, patch
 
 
-client = TestClient(app)
 
-def test_discovery_endpoint():
+def test_discovery_endpoint(client):
     # Mock the service to avoid DB calls
     with patch('src.api.discovery.SegmentDiscoveryService') as MockService:
         instance = MockService.return_value
@@ -25,7 +24,7 @@ def test_discovery_endpoint():
         assert isinstance(data["debug_paths"], list)
 
         
-def test_discovery_page_render():
+def test_discovery_page_render(client):
     response = client.get("/discovery")
     assert response.status_code == 200
     assert "Segment Discovery" in response.text