feat: introduce public_id UUID field for user identification in URLs and APIs

backend/users/migrations/0003_user_public_id.py

@@ -0,0 +1,19 @@
+# Generated by Django 6.0.4 on 2026-04-10 11:39
+
+import uuid
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("users", "0002_user_kdf_salt_alter_user_is_active"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="user",
+            name="public_id",
+            field=models.UUIDField(default=uuid.uuid4, editable=False, null=True),
+        ),
+    ]