chore: apply phase 0 review fixes

apps/users/migrations/0002_alter_user_email.py

@@ -0,0 +1,18 @@
+# Generated by Django 5.2.15 on 2026-07-02 02:11
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('users', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='user',
+            name='email',
+            field=models.EmailField(max_length=254, unique=True, verbose_name='email address'),
+        ),
+    ]

apps/users/models.py

@@ -7,7 +7,7 @@ class User(AbstractUser):
         ACTIVE = "active", "Active"
         DISABLED = "disabled", "Disabled"
 
-    email = models.EmailField("email address")
+    email = models.EmailField("email address", unique=True)
     payment_user_id = models.CharField(
         max_length=128,
         blank=True,