Add body battery: sync, storage, and health UI chart

Parses Garmin Connect get_body_battery() per day, storing charged/drained/
start+end levels and the fine-grained [[ts_ms, level, type, stress]] values
array in a new body_battery JSONB column on health_metrics.

Frontend adds:
- BatteryRing SVG gauge (color-scaled 0–100)
- BodyBatteryChart: ComposedChart with type-colored bars (REST/ACTIVE/SLEEP/
  STRESS) and battery level overlay line, matching Garmin's layout
- Body battery trend chart in the Trends section (end_level per day)

Also adds avg_hr_day and weight data which now correctly sync with the
intraday_hr JSON serialization fix from the previous commit.

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

backend/app/api/health.py

@@ -1,8 +1,8 @@
 from fastapi import APIRouter, Depends, Query
 from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy import select, desc, func
-from pydantic import BaseModel
-from typing import Optional, List
+from pydantic import BaseModel, model_validator
+from typing import Optional, List, Any
 from datetime import datetime, timedelta, timezone
 
 from app.core.database import get_db
@@ -44,6 +44,13 @@ class HealthMetricOut(BaseModel):
     active_calories: Optional[float]
     total_calories: Optional[float]
     spo2_avg: Optional[float]
+    body_battery: Optional[Any] = None   # {charged,drained,start_level,end_level} — values stripped
+
+    @model_validator(mode='after')
+    def _strip_bb_values(self):
+        if isinstance(self.body_battery, dict):
+            self.body_battery = {k: v for k, v in self.body_battery.items() if k != 'values'}
+        return self
 
     class Config:
         from_attributes = True
@@ -136,7 +143,10 @@ async def intraday_health(
         )
     )
     metric = result.scalar_one_or_none()
-    return {"hr_values": metric.intraday_hr if metric else None}
+    return {
+        "hr_values":    metric.intraday_hr if metric else None,
+        "body_battery": metric.body_battery if metric else None,
+    }
 
 
 @router.put("/manual")