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/workers/tasks.py

@@ -439,4 +439,29 @@ def process_garmin_health_zip(zip_path: str, user_id: int):
                         "spo2": data.get("avgSpo2"),
                     })
 
-        db.commit()
+        db.commit()
+
+
+@celery_app.task(name="recalculate_hr_zones_for_user")
+def recalculate_hr_zones_for_user(user_id: int, new_max_hr: float):
+    """Recalculate hr_zones for all of a user's activities using a new max HR."""
+    from app.services.fit_parser import calculate_hr_zones
+    from app.core.database import SyncSessionLocal
+    from app.models.user import Activity, ActivityDataPoint
+    from sqlalchemy import select
+
+    with SyncSessionLocal() as db:
+        activities = db.execute(
+            select(Activity).where(Activity.user_id == user_id)
+        ).scalars().all()
+
+        for activity in activities:
+            data_points = db.execute(
+                select(ActivityDataPoint).where(ActivityDataPoint.activity_id == activity.id)
+            ).scalars().all()
+            points_dicts = [{"heart_rate": dp.heart_rate} for dp in data_points]
+            new_zones = calculate_hr_zones(points_dicts, new_max_hr)
+            if new_zones:
+                activity.hr_zones = new_zones
+
+        db.commit()