Health hypnogram, routes tiles, BB bar chart, segment delta

- Sleep: store per-epoch stage timestamps in new sleep_stages JSON column;
  DailySnapshot now renders a proper 4-lane hypnogram (Awake/REM/Light/Deep)
  instead of the old proportional flat bar
- Body battery: replace grey background bars + white line with per-minute bars
  coloured by inferred type (sleep=indigo, rest=teal, active=orange, stable=grey)
  derived from sleep window + battery direction; Y-axis fixed 0-100
- Routes: convert sidebar list to tile grid sorted by most completions; tiles
  colour-bordered by sport type (blue=running, orange=cycling); completion count
  shown on each tile; detail panel displays below the grid when a tile is clicked
- Segments on activity detail: add column headers (This run / Best / Δ) and
  show signed time delta vs best, green when faster, red when slower

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

backend/app/main.py

@@ -58,6 +58,7 @@ async def init_db():
                 "ALTER TABLE health_metrics ADD COLUMN IF NOT EXISTS max_hr_day FLOAT",
                 "ALTER TABLE health_metrics ADD COLUMN IF NOT EXISTS intraday_hr JSONB",
                 "ALTER TABLE health_metrics ADD COLUMN IF NOT EXISTS body_battery JSONB",
+                "ALTER TABLE health_metrics ADD COLUMN IF NOT EXISTS sleep_stages JSON",
             ]:
                 await conn.execute(text(stmt))
     except Exception as e: