Add segments, YTD stats, route matching fixes, body battery layout, pace fix

- Segments page: new /segments route with auto-generate (1km splits, turn
  detection, hill detection), manual segment creation, per-segment performance
  times across matched activities; fixed auth on existing segment endpoints
- YTD distance: new /activities/stats/ytd endpoint; Dashboard replaces
  'Total distance' with 'Running this year' + 'Cycling this year'; Activities
  page shows YTD stats row
- Weekly chart click: clicking a Dashboard bar navigates to Activities filtered
  to that week; Activities reads from/to query params with dismissable chip
- Route matching: add ±2.5% distance gate + 3% relative DTW threshold
  (was flat 80m); tighten candidate pre-filter from 80/120% to 95/105%
- Body battery layout: HR chart and body battery now side-by-side at same
  height on large screens instead of stacked full-width
- Pace display fix: MetricTimeline clamps GPS speed outliers before computing
  Y-axis domain; tick formatter guards against v<=0 or v>25 m/s

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/main.py

@@ -63,6 +63,15 @@ async def init_db():
     except Exception as e:
         print(f"health_metrics column migration skipped: {e}")
 
+    # route_segments auto_generated column added after initial creation
+    try:
+        async with engine.begin() as conn:
+            await conn.execute(text(
+                "ALTER TABLE route_segments ADD COLUMN IF NOT EXISTS auto_generated BOOLEAN DEFAULT FALSE"
+            ))
+    except Exception as e:
+        print(f"route_segments column migration skipped: {e}")
+
     # Replace the all-columns unique constraint on personal_records with a partial
     # index (only current records must be unique per user/sport/distance).
     # The old constraint also covered is_current_record=False rows, causing