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/migrations/0003_userhearts_full_refill_at.py

@@ -0,0 +1,18 @@
+# Generated by Django 5.2.17 on 2026-09-24 03:23
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('progress', '0002_character_userstats_selected_character_id'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='userhearts',
+            name='full_refill_at',
+            field=models.DateTimeField(blank=True, null=True),
+        ),
+    ]