feat: add Eventify ID (EVT-XXXXXXXX) to User model and all APIs

- Add eventify_id CharField (unique, indexed, editable=False) to User
- Auto-generate on save() with charset excluding I/O/0/1 for clarity
- Migration 0012: add field nullable, backfill all existing users, make non-null
- Sync migration 0011 (allowed_modules) pulled from server
- Expose eventify_id in accounts/api.py, partner/api.py serializers
- Expose eventify_id in mobile_api login response (populates localStorage)

accounts/migrations/0012_user_eventify_id.py

@@ -0,0 +1,55 @@
+# Generated migration for eventify_id field
+
+import secrets
+
+from django.db import migrations, models
+
+EVENTIFY_ID_CHARS = 'ABCDEFGHJKLMNPQRSTUVWXYZ23456789'
+
+
+def generate_unique_eventify_id(existing_ids):
+    for _ in range(100):
+        candidate = 'EVT-' + ''.join(secrets.choice(EVENTIFY_ID_CHARS) for _ in range(8))
+        if candidate not in existing_ids:
+            return candidate
+    raise RuntimeError("Could not generate a unique Eventify ID after 100 attempts")
+
+
+def backfill_eventify_ids(apps, schema_editor):
+    User = apps.get_model('accounts', 'User')
+    existing_ids = set(User.objects.exclude(eventify_id__isnull=True).values_list('eventify_id', flat=True))
+    users_to_update = []
+    for user in User.objects.filter(eventify_id__isnull=True):
+        new_id = generate_unique_eventify_id(existing_ids)
+        existing_ids.add(new_id)
+        user.eventify_id = new_id
+        users_to_update.append(user)
+    User.objects.bulk_update(users_to_update, ['eventify_id'])
+
+
+def reverse_backfill(apps, schema_editor):
+    pass  # No-op: reversing just leaves IDs set, which is fine
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('accounts', '0011_user_allowed_modules_alter_user_id'),
+    ]
+
+    operations = [
+        # Step 1: Add the column as nullable
+        migrations.AddField(
+            model_name='user',
+            name='eventify_id',
+            field=models.CharField(blank=True, db_index=True, editable=False, max_length=12, null=True, unique=True),
+        ),
+        # Step 2: Backfill all existing users
+        migrations.RunPython(backfill_eventify_ids, reverse_backfill),
+        # Step 3: Make the field non-nullable
+        migrations.AlterField(
+            model_name='user',
+            name='eventify_id',
+            field=models.CharField(blank=False, db_index=True, editable=False, max_length=12, null=False, unique=True),
+        ),
+    ]