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>

subscription/migrations/0009_question_packs.py

@@ -0,0 +1,58 @@
+# Generated by Django 5.2.17 on 2026-10-02 07:20
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('subscription', '0008_paid_quota_ten_a_day'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='HeartsPass',
+            fields=[
+                ('id', models.BigAutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('user_id', models.IntegerField(db_index=True, unique=True)),
+                ('expires_at', models.DateTimeField()),
+            ],
+        ),
+        migrations.AddField(
+            model_name='plan',
+            name='hearts_days',
+            field=models.PositiveSmallIntegerField(default=0, help_text='فقط بسته: چند روز قلبِ نامحدود هدیه.'),
+        ),
+        migrations.AddField(
+            model_name='plan',
+            name='kind',
+            field=models.CharField(choices=[('pack', 'بسته\u200cی سوال'), ('subscription', 'اشتراکِ زمانی')], default='subscription', max_length=15),
+        ),
+        migrations.AddField(
+            model_name='plan',
+            name='on_sale',
+            field=models.BooleanField(default=True, help_text='در صفحه\u200cی خرید نشان داده شود.'),
+        ),
+        migrations.AddField(
+            model_name='plan',
+            name='question_credits',
+            field=models.PositiveIntegerField(default=0, help_text='فقط بسته: چند سوال از دستیار به اعتبارِ خریدار اضافه می\u200cشود.'),
+        ),
+        migrations.CreateModel(
+            name='QuestionCredit',
+            fields=[
+                ('id', models.BigAutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('user_id', models.IntegerField(db_index=True)),
+                ('delta', models.IntegerField()),
+                ('reason', models.CharField(choices=[('purchase', 'خرید'), ('question', 'سوال'), ('grant', 'هدیه\u200cی ادمین')], max_length=10)),
+                ('order_id', models.IntegerField(blank=True, null=True)),
+                ('message_id', models.IntegerField(blank=True, db_index=True, null=True)),
+                ('note', models.CharField(blank=True, max_length=100)),
+                ('created_at', models.DateTimeField(auto_now_add=True)),
+            ],
+            options={
+                'ordering': ['-created_at'],
+                'constraints': [models.UniqueConstraint(condition=models.Q(('reason', 'purchase')), fields=('order_id',), name='uniq_credit_per_paid_order')],
+            },
+        ),
+    ]