Fix upload auto-refresh, health data refresh, and HR zone recalculation

- UploadPage now polls task status every 2s and invalidates activity,
  health-summary, and health-metrics queries on completion so new
  activities and health data appear without a hard refresh
- Garmin and Strava export endpoints now return a task_id for polling
- Updating max HR in Profile triggers a background Celery task to
  recalculate hr_zones for all existing activities; profile page shows
  a confirmation note when this is queued
- Add CLAUDE.md with repo architecture and dev commands

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

backend/app/api/profile.py

@@ -55,6 +55,7 @@ async def update_profile(
     db: AsyncSession = Depends(get_db),
     current_user: User = Depends(get_current_user),
 ):
+    old_max_hr = current_user.max_heart_rate
     if body.max_heart_rate is not None:
         if not (100 <= body.max_heart_rate <= 250):
             raise HTTPException(400, "Max HR must be 100–250")
@@ -74,6 +75,11 @@ async def update_profile(
 
     await db.commit()
     await db.refresh(current_user)
+
+    if body.max_heart_rate is not None and body.max_heart_rate != old_max_hr:
+        from app.workers.tasks import recalculate_hr_zones_for_user
+        recalculate_hr_zones_for_user.delay(current_user.id, body.max_heart_rate)
+
     return {**{c.name: getattr(current_user, c.name)
                for c in User.__table__.columns},
             "estimated_max_hr": _estimated_max_hr(current_user)}