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/upload.py

@@ -82,7 +82,7 @@ async def upload_garmin_export(
     return {
         "status": "queued",
         "activity_tasks": len(task_ids),
-        "health_task": health_task.id,
+        "task_id": health_task.id,
     }
 
 
@@ -116,6 +116,7 @@ async def upload_strava_export(
     return {
         "status": "queued",
         "activity_tasks": len(task_ids),
+        "task_id": task_ids[-1] if task_ids else None,
     }