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/internal/handler/wire.go

@@ -23,6 +23,7 @@ func ProvideAdminHandlers(
 	systemHandler *admin.SystemHandler,
 	subscriptionHandler *admin.SubscriptionHandler,
 	usageHandler *admin.UsageHandler,
+	userAttributeHandler *admin.UserAttributeHandler,
 ) *AdminHandlers {
 	return &AdminHandlers{
 		Dashboard:        dashboardHandler,
@@ -39,6 +40,7 @@ func ProvideAdminHandlers(
 		System:           systemHandler,
 		Subscription:     subscriptionHandler,
 		Usage:            usageHandler,
+		UserAttribute:    userAttributeHandler,
 	}
 }
 
@@ -107,6 +109,7 @@ var ProviderSet = wire.NewSet(
 	ProvideSystemHandler,
 	admin.NewSubscriptionHandler,
 	admin.NewUsageHandler,
+	admin.NewUserAttributeHandler,
 
 	// AdminHandlers and Handlers constructors
 	ProvideAdminHandlers,