✨(back) add subscription migrations

Add database migrations for CalendarSubscriptionToken model
and create index on token+is_active for efficient lookups.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/backend/core/migrations/0002_calendarsubscriptiontoken.py

@@ -0,0 +1,34 @@
+# Generated by Django 5.2.9 on 2026-01-25 14:21
+
+import django.db.models.deletion
+import uuid
+from django.conf import settings
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('core', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='CalendarSubscriptionToken',
+            fields=[
+                ('id', models.UUIDField(default=uuid.uuid4, editable=False, primary_key=True, serialize=False)),
+                ('caldav_path', models.CharField(help_text='CalDAV path of the calendar', max_length=512)),
+                ('calendar_name', models.CharField(blank=True, default='', help_text='Display name of the calendar', max_length=255)),
+                ('token', models.UUIDField(db_index=True, default=uuid.uuid4, help_text='Secret token used in the subscription URL', unique=True)),
+                ('is_active', models.BooleanField(default=True, help_text='Whether this subscription token is active')),
+                ('last_accessed_at', models.DateTimeField(blank=True, help_text='Last time this subscription URL was accessed', null=True)),
+                ('created_at', models.DateTimeField(auto_now_add=True)),
+                ('owner', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='subscription_tokens', to=settings.AUTH_USER_MODEL)),
+            ],
+            options={
+                'verbose_name': 'calendar subscription token',
+                'verbose_name_plural': 'calendar subscription tokens',
+                'constraints': [models.UniqueConstraint(fields=('owner', 'caldav_path'), name='unique_token_per_owner_calendar')],
+            },
+        ),
+    ]

src/backend/core/migrations/0003_calendarsubscriptiontoken_token_active_idx.py

@@ -0,0 +1,17 @@
+# Generated by Django 5.2.9 on 2026-01-25 15:40
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('core', '0002_calendarsubscriptiontoken'),
+    ]
+
+    operations = [
+        migrations.AddIndex(
+            model_name='calendarsubscriptiontoken',
+            index=models.Index(fields=['token', 'is_active'], name='token_active_idx'),
+        ),
+    ]