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>

frontend/src/pages/ProfilePage.jsx

@@ -1,4 +1,4 @@
-import { useState, useEffect } from 'react'
+import { useState, useEffect, useRef } from 'react'
 import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query'
 import api from '../utils/api'
 import { useAuthStore } from '../hooks/useAuth'
@@ -59,6 +59,8 @@ export default function ProfilePage() {
   // HR / measurements form
   const [hrForm, setHrForm] = useState({ max_heart_rate: '', resting_heart_rate: '', birth_year: '', height_cm: '' })
   const [hrSaved, setHrSaved] = useState(false)
+  const [hrZoneRecalc, setHrZoneRecalc] = useState(false)
+  const maxHrChangedRef = useRef(false)
   useEffect(() => {
     if (profile) setHrForm({
       max_heart_rate: profile.max_heart_rate || '',
@@ -70,7 +72,16 @@ export default function ProfilePage() {
 
   const updateProfile = useMutation({
     mutationFn: data => api.patch('/profile/', data).then(r => r.data),
-    onSuccess: () => { qc.invalidateQueries({ queryKey: ['profile'] }); setHrSaved(true); setTimeout(() => setHrSaved(false), 3000) },
+    onSuccess: () => {
+      qc.invalidateQueries({ queryKey: ['profile'] })
+      setHrSaved(true)
+      setTimeout(() => setHrSaved(false), 3000)
+      if (maxHrChangedRef.current) {
+        setHrZoneRecalc(true)
+        setTimeout(() => setHrZoneRecalc(false), 6000)
+        maxHrChangedRef.current = false
+      }
+    },
   })
 
   // Weight log
@@ -149,12 +160,19 @@ export default function ProfilePage() {
         </div>
 
         <SaveButton
-          onClick={() => updateProfile.mutate(Object.fromEntries(
-            Object.entries(hrForm).filter(([,v]) => v !== '').map(([k,v]) => [k, parseFloat(v)])
-          ))}
+          onClick={() => {
+            const data = Object.fromEntries(
+              Object.entries(hrForm).filter(([,v]) => v !== '').map(([k,v]) => [k, parseFloat(v)])
+            )
+            maxHrChangedRef.current = data.max_heart_rate !== undefined && data.max_heart_rate !== profile?.max_heart_rate
+            updateProfile.mutate(data)
+          }}
           loading={updateProfile.isPending}
           saved={hrSaved}
         />
+        {hrZoneRecalc && (
+          <p className="text-xs text-blue-400 mt-1">HR zones are being recalculated for your existing activities.</p>
+        )}
       </Section>
 
       {/* Weight log */}