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_repro_segment_bug.py

@@ -0,0 +1,72 @@
+
+from src.models.activity import Activity
+import pytest
+
+def test_repro_segment_wrong_type(client, db_session):
+    """
+    Reproduction: Create a segment from a 'running' activity and verify it is not saved as 'cycling'.
+    """
+    # 1. Setup a fake running activity in DB
+    act_id = 999999999
+    
+    # Dummy TCX
+    dummy_tcx = """
+    <TrainingCenterDatabase>
+      <Activities>
+        <Activity Sport="Running">
+          <Id>2018-01-01T00:00:00Z</Id>
+          <Lap StartTime="2018-01-01T00:00:00Z">
+            <Track>
+              <Trackpoint>
+                <Time>2018-01-01T00:00:00Z</Time>
+                <Position>
+                  <LatitudeDegrees>45.0</LatitudeDegrees>
+                  <LongitudeDegrees>-33.0</LongitudeDegrees>
+                </Position>
+              </Trackpoint>
+              <Trackpoint>
+                <Time>2018-01-01T00:00:10Z</Time>
+                <Position>
+                  <LatitudeDegrees>45.01</LatitudeDegrees>
+                  <LongitudeDegrees>-33.01</LongitudeDegrees>
+                </Position>
+              </Trackpoint>
+            </Track>
+          </Lap>
+        </Activity>
+      </Activities>
+    </TrainingCenterDatabase>
+    """
+    
+    act = Activity(
+        id=act_id,
+        garmin_activity_id=str(act_id), 
+        activity_name="Test Run", 
+        activity_type="running", # Correct Type in DB
+        file_content=dummy_tcx.encode('utf-8'),
+        file_type="tcx"
+    )
+    db_session.add(act)
+    db_session.commit()
+
+    # 2. Call Create Segment Endpoint
+    payload = {
+        "activity_id": act_id,
+        "name": "Test Segment",
+        "start_index": 0,
+        "end_index": 1,
+        "activity_type": "cycling" # Simulate Frontend forcing 'cycling'
+    }
+    
+    response = client.post("/api/segments/create", json=payload)
+    assert response.status_code == 200, f"Response: {response.text}"
+    
+    data = response.json()
+    seg_id = data['id']
+    from src.models.segment import Segment
+    segment = db_session.query(Segment).filter(Segment.id == seg_id).first()
+    
+    print(f"Created Segment Type: {segment.activity_type}")
+    
+    # Assert it is running
+    assert segment.activity_type == 'running'