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/integration/test_discovery_fix.py

@@ -0,0 +1,55 @@
+import pytest
+from src.models.activity import Activity
+from src.services.parsers import extract_activity_data
+import os
+
+@pytest.mark.asyncio
+async def test_discovery_returns_file_type_when_db_type_missing(db_session, client, tmp_path):
+    """
+    Verify that the discovery API returns the activity type parsed from the file
+    even if the database record has activity_type=None.
+    """
+    # 1. Create dummy FIT file content (minimal valid header/data or mock)
+    # Using a real file is better, or mocking extract_activity_data. 
+    # Let's mock extract_activity_data to avoid needing a complex binary file.
+    
+    from unittest.mock import patch
+    
+    # Mock return value mimics a running activity
+    mock_parsed_data = {
+        'type': 'running',
+        'points': [[-122.4, 37.8], [-122.41, 37.81]], # Dummy points
+        'timestamps': []
+    }
+    
+    with patch('src.api.discovery.extract_activity_data', return_value=mock_parsed_data) as mock_extract:
+        # 2. Create Activity in DB with type=None
+        activity = Activity(
+            activity_name="Test Activity",
+            garmin_activity_id="99999",
+            start_time="2023-01-01T10:00:00",
+            activity_type=None, # <--- MISSING TYPE
+            file_type="fit",
+            file_content=b"dummy_bytes"
+        )
+        db_session.add(activity)
+        db_session.commit()
+        db_session.refresh(activity)
+        
+        # 3. Call Discovery Single API
+        payload = {
+            "activity_id": activity.id,
+            "pause_threshold": 10,
+            "rdp_epsilon": 10,
+            "turn_threshold": 60,
+            "min_length": 100
+        }
+        
+        response = client.post("/api/discovery/single", json=payload)
+        
+        assert response.status_code == 200
+        data = response.json()
+        
+        # 4. Assert analyzed_activity_type is 'running'
+        assert data['analyzed_activity_type'] == 'running'
+        print("Success: API returned inferred type 'running'")