feat(backend): add user custom attributes system

Add a flexible user attribute system that allows admins to define
custom fields for users (text, textarea, number, email, url, date,
select, multi_select types).

- Add Ent schemas for UserAttributeDefinition and UserAttributeValue
- Add service layer with validation logic
- Add repository layer with batch operations support
- Add admin API endpoints for CRUD and reorder operations
- Add batch API for loading attribute values for multiple users
- Add database migration (018_user_attributes.sql)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

backend/ent/hook/hook.go

@@ -129,6 +129,30 @@ func (f UserAllowedGroupFunc) Mutate(ctx context.Context, m ent.Mutation) (ent.V
 	return nil, fmt.Errorf("unexpected mutation type %T. expect *ent.UserAllowedGroupMutation", m)
 }
 
+// The UserAttributeDefinitionFunc type is an adapter to allow the use of ordinary
+// function as UserAttributeDefinition mutator.
+type UserAttributeDefinitionFunc func(context.Context, *ent.UserAttributeDefinitionMutation) (ent.Value, error)
+
+// Mutate calls f(ctx, m).
+func (f UserAttributeDefinitionFunc) Mutate(ctx context.Context, m ent.Mutation) (ent.Value, error) {
+	if mv, ok := m.(*ent.UserAttributeDefinitionMutation); ok {
+		return f(ctx, mv)
+	}
+	return nil, fmt.Errorf("unexpected mutation type %T. expect *ent.UserAttributeDefinitionMutation", m)
+}
+
+// The UserAttributeValueFunc type is an adapter to allow the use of ordinary
+// function as UserAttributeValue mutator.
+type UserAttributeValueFunc func(context.Context, *ent.UserAttributeValueMutation) (ent.Value, error)
+
+// Mutate calls f(ctx, m).
+func (f UserAttributeValueFunc) Mutate(ctx context.Context, m ent.Mutation) (ent.Value, error) {
+	if mv, ok := m.(*ent.UserAttributeValueMutation); ok {
+		return f(ctx, mv)
+	}
+	return nil, fmt.Errorf("unexpected mutation type %T. expect *ent.UserAttributeValueMutation", m)
+}
+
 // The UserSubscriptionFunc type is an adapter to allow the use of ordinary
 // function as UserSubscription mutator.
 type UserSubscriptionFunc func(context.Context, *ent.UserSubscriptionMutation) (ent.Value, error)