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/tx.go

@@ -34,6 +34,10 @@ type Tx struct {
 	User *UserClient
 	// UserAllowedGroup is the client for interacting with the UserAllowedGroup builders.
 	UserAllowedGroup *UserAllowedGroupClient
+	// UserAttributeDefinition is the client for interacting with the UserAttributeDefinition builders.
+	UserAttributeDefinition *UserAttributeDefinitionClient
+	// UserAttributeValue is the client for interacting with the UserAttributeValue builders.
+	UserAttributeValue *UserAttributeValueClient
 	// UserSubscription is the client for interacting with the UserSubscription builders.
 	UserSubscription *UserSubscriptionClient
 
@@ -177,6 +181,8 @@ func (tx *Tx) init() {
 	tx.UsageLog = NewUsageLogClient(tx.config)
 	tx.User = NewUserClient(tx.config)
 	tx.UserAllowedGroup = NewUserAllowedGroupClient(tx.config)
+	tx.UserAttributeDefinition = NewUserAttributeDefinitionClient(tx.config)
+	tx.UserAttributeValue = NewUserAttributeValueClient(tx.config)
 	tx.UserSubscription = NewUserSubscriptionClient(tx.config)
 }