refill hearts like Duolingo: one every 5 hours, all of them after 24

Each heart used to take 24 hours, so a student who lost all five waited up to
five days. Now one heart returns every 5 hours, and 24 hours after the first
heart was lost they all come back at once (the owner's call). /api/user/hearts
reports both times (nextRefillAt, fullRefillAt); an existing timer longer than
5 hours is shortened the first time it is read.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

progress/admin.py

@@ -33,4 +33,4 @@ class StreakRecordAdmin(admin.ModelAdmin):
 
 @admin.register(UserHearts)
 class UserHeartsAdmin(admin.ModelAdmin):
-    list_display = ('id', 'user_id', 'current_hearts', 'max_hearts', 'next_refill_at')
+    list_display = ('id', 'user_id', 'current_hearts', 'max_hearts', 'next_refill_at', 'full_refill_at')