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_ui_rendering.py

@@ -0,0 +1,74 @@
+
+from fastapi.testclient import TestClient
+from main import app
+import pytest
+
+def test_activity_view_render(client):
+    """
+    Test that the activity view page renders the HTML shell correctly 
+    and contains the necessary container elements for JS to populate.
+    """
+    # Requires an activity ID path parameter, but doesn't validate it in the route handler
+    response = client.get("/activity/12345")
+    
+    assert response.status_code == 200
+    html = response.text
+    
+    # Verify Title/Header placeholders
+    assert 'id="act-name"' in html
+    assert 'id="act-time"' in html
+    
+    # Verify Map Container
+    assert 'id="map"' in html
+    
+    # Verify Chart Container
+    assert 'id="streams-chart"' in html
+    
+    # Verify Stats Grid
+    assert 'class="stats-grid"' in html
+    assert 'id="metric-dist"' in html
+    assert 'id="metric-dur"' in html
+    
+    # Verify Metrics Section
+    assert 'class="metrics-section"' in html
+    assert 'id="m-avg-pwr"' in html
+
+def test_segment_view_render(client):
+    """
+    Test that the segments page renders correctly with the segments table
+    and hidden modals for viewing and comparing segments.
+    """
+    response = client.get("/segments")
+    
+    assert response.status_code == 200
+    html = response.text
+    
+    # Verify Page Title
+    assert "Segments" in html
+    
+    # Verify Segments Table
+    assert 'id="segments-table"' in html
+    
+    # Verify Create Modal Trigger
+    assert 'data-bs-target="#createSegmentModal"' in html
+
+def test_comparison_modals_render(client):
+    """
+    Test that the Segment Comparison and Details modals are present in the DOM
+    of the segments page.
+    """
+    response = client.get("/segments")
+    assert response.status_code == 200
+    html = response.text
+    
+    # Verify View Modal & Attributes
+    assert 'id="viewSegmentModal"' in html
+    assert 'id="seg-map"' in html
+    assert 'id="elevationChart"' in html
+    assert 'id="leaderboard-table"' in html
+    
+    # Verify Comparison Modal & Attributes
+    assert 'id="compareModal"' in html
+    assert 'Effort Comparison' in html
+    assert 'id="comparison-table"' in html
+    assert 'id="comparisonChart"' in html