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_single_discovery.py

@@ -2,9 +2,8 @@ from fastapi.testclient import TestClient
 from main import app
 from unittest.mock import MagicMock, patch
 
-client = TestClient(app)
 
-def test_single_discovery_endpoint():
+def test_single_discovery_endpoint(client):
     # Mock the service
     with patch('src.api.discovery.SegmentDiscoveryService') as MockService:
         instance = MockService.return_value
@@ -29,7 +28,7 @@ def test_single_discovery_endpoint():
         assert data["candidates"][0]["frequency"] == 1
         assert data["candidates"][0]["distance"] == 1000.0
 
-def test_single_discovery_not_found():
+def test_single_discovery_not_found(client):
      with patch('src.api.discovery.SegmentDiscoveryService') as MockService:
         instance = MockService.return_value
         instance.analyze_single_activity.return_value = []