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/models/user.py

@@ -248,6 +248,7 @@ class HealthMetric(Base):
     intraday_hr = Column(JSON, nullable=True)           # [[epoch_ms, bpm], ...] — not in API list response
     body_battery = Column(JSON, nullable=True)          # {charged,drained,start_level,end_level,values:[[ts_ms,level,type,stress]...]}
     body_battery_hires = Column(JSON, nullable=True)    # [[ts_ms, level], ...] interpolated from bb + HR; higher resolution than raw values
+    sleep_stages = Column(JSON, nullable=True)          # [[ts_ms, level], ...] 0=unmeasurable,1=awake,2=light,3=deep,4=rem
 
     __table_args__ = (
         UniqueConstraint("user_id", "date", name="uq_health_user_date"),