question packs instead of monthly/yearly plans; hearts are real

- packs (20 / 60 / 150 questions for 45,000 / 119,000 / 259,000 toman) never expire and
  give 7 / 30 / 60 days of unlimited hearts; old subscription plans stay for existing
  subscribers but are no longer on sale
- credit ledger (subscription.QuestionCredit): +N per paid pack (once per order), -1 per
  tutor question that reaches the API; the free daily questions are spent first; questions
  stopped by the server filter, cached answers and failed answers cost nothing
- unlimited hearts from packs live in HeartsPass, so buying a pack never rewrites a
  running subscription
- Message.charge records how each question was paid; GET /api/ai/quota and the ask
  responses report free-left-today and credits
- HEARTS_ENFORCED now defaults to on: zero hearts stops answering until refill or a pack

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

ai_tutor/migrations/0007_message_charge.py

@@ -0,0 +1,25 @@
+# Generated by Django 5.2.17 on 2026-10-02 07:34
+
+from django.db import migrations, models
+
+
+def mark_existing_as_free(apps, schema_editor):
+    # پیام‌های پیش از بسته‌ها همه از سهمیه‌ی روزانه بودند؛ بدونِ این، سوال‌های امروزِ پیش از
+    # دیپلو در شمارشِ سهمیه نمی‌آمدند.
+    apps.get_model('ai_tutor', 'Message').objects.filter(role='user').update(charge='free')
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('ai_tutor', '0006_message_diagram'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='message',
+            name='charge',
+            field=models.CharField(blank=True, choices=[('free', 'رایگانِ روزانه'), ('credit', 'اعتبارِ بسته'), ('none', 'بی\u200cهزینه')], default='', max_length=6),
+        ),
+        migrations.RunPython(mark_existing_as_free, migrations.RunPython.noop),
+    ]