feat(notifications): add scheduled email notification system

- NotificationSchedule + NotificationRecipient models with initial migration
- emails.py BUILDERS registry + events_expiring_this_week HTML email builder (IST week bounds)
- send_scheduled_notifications management command (croniter due-check + select_for_update(skip_locked))
- 6 admin API endpoints under /api/v1/notifications/ (types, schedules CRUD, recipients CRUD, send-now)
- date_from/date_to filters on EventListView for dashboard card
- croniter>=2.0.0 added to requirements

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

notifications/migrations/0001_initial.py

@@ -0,0 +1,93 @@
+from django.conf import settings
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    initial = True
+
+    dependencies = [
+        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='Notification',
+            fields=[
+                ('id', models.BigAutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('title', models.CharField(max_length=255)),
+                ('message', models.TextField()),
+                ('notification_type', models.CharField(
+                    choices=[
+                        ('event', 'Event'),
+                        ('promo', 'Promotion'),
+                        ('system', 'System'),
+                        ('booking', 'Booking'),
+                    ],
+                    default='system', max_length=20,
+                )),
+                ('is_read', models.BooleanField(default=False)),
+                ('action_url', models.URLField(blank=True, null=True)),
+                ('created_at', models.DateTimeField(auto_now_add=True)),
+                ('user', models.ForeignKey(
+                    on_delete=django.db.models.deletion.CASCADE,
+                    related_name='notifications',
+                    to=settings.AUTH_USER_MODEL,
+                )),
+            ],
+            options={'ordering': ['-created_at']},
+        ),
+        migrations.CreateModel(
+            name='NotificationSchedule',
+            fields=[
+                ('id', models.BigAutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('name', models.CharField(max_length=200)),
+                ('notification_type', models.CharField(
+                    choices=[('events_expiring_this_week', 'Events Expiring This Week')],
+                    db_index=True, max_length=64,
+                )),
+                ('cron_expression', models.CharField(
+                    default='0 0 * * 1',
+                    help_text=(
+                        'Standard 5-field cron (minute hour dom month dow). '
+                        'Evaluated in Asia/Kolkata.'
+                    ),
+                    max_length=100,
+                )),
+                ('is_active', models.BooleanField(db_index=True, default=True)),
+                ('last_run_at', models.DateTimeField(blank=True, null=True)),
+                ('last_status', models.CharField(blank=True, default='', max_length=20)),
+                ('last_error', models.TextField(blank=True, default='')),
+                ('created_at', models.DateTimeField(auto_now_add=True)),
+                ('updated_at', models.DateTimeField(auto_now=True)),
+            ],
+            options={'ordering': ['-created_at']},
+        ),
+        migrations.AddIndex(
+            model_name='notificationschedule',
+            index=models.Index(
+                fields=['is_active', 'notification_type'],
+                name='notificatio_is_acti_26dfb5_idx',
+            ),
+        ),
+        migrations.CreateModel(
+            name='NotificationRecipient',
+            fields=[
+                ('id', models.BigAutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('email', models.EmailField(max_length=254)),
+                ('display_name', models.CharField(blank=True, default='', max_length=200)),
+                ('is_active', models.BooleanField(default=True)),
+                ('created_at', models.DateTimeField(auto_now_add=True)),
+                ('schedule', models.ForeignKey(
+                    on_delete=django.db.models.deletion.CASCADE,
+                    related_name='recipients',
+                    to='notifications.notificationschedule',
+                )),
+            ],
+            options={
+                'ordering': ['display_name', 'email'],
+                'unique_together': {('schedule', 'email')},
+            },
+        ),
+    ]