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_garmin_id.py

@@ -0,0 +1,45 @@
+import pytest
+from src.models.activity import Activity
+import os
+
+@pytest.mark.asyncio
+async def test_discovery_by_garmin_id(db_session, client, tmp_path):
+    """
+    Verify that the discovery API correctly finds an activity when passed its
+    Garmin Activity ID (as an int/string) instead of the internal DB ID.
+    """
+    # 1. Create Activity with a specific Garmin ID
+    garmin_id = "9876543210" # Large ID
+    activity = Activity(
+        activity_name="Garmin Test Activity",
+        garmin_activity_id=garmin_id,
+        start_time="2023-05-20T10:00:00",
+        activity_type="hiking", 
+        file_type="fit",
+        file_content=b"dummy"
+    )
+    db_session.add(activity)
+    db_session.commit()
+    db_session.refresh(activity)
+    
+    # Internal ID should be small (e.g. 1)
+    print(f"DEBUG: Created Activity Internal ID: {activity.id}, Garmin ID: {activity.garmin_activity_id}")
+    
+    # 2. Call Discovery Single API with the GARMIN ID
+    payload = {
+        "activity_id": int(garmin_id), # Passing the large ID
+        "pause_threshold": 10,
+        "rdp_epsilon": 10,
+        "turn_threshold": 60,
+        "min_length": 100
+    }
+    
+    response = client.post("/api/discovery/single", json=payload)
+    
+    # Needs to be 200 OK
+    assert response.status_code == 200
+    data = response.json()
+    
+    # 3. Assert analyzed_activity_type is 'hiking' (retrieved from DB via Garmin ID lookup)
+    assert data['analyzed_activity_type'] == 'hiking'
+    print("Success: API found activity via Garmin ID and returned type 'hiking'")