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/handler.go

@@ -20,6 +20,7 @@ type AdminHandlers struct {
 	System           *admin.SystemHandler
 	Subscription     *admin.SubscriptionHandler
 	Usage            *admin.UsageHandler
+	UserAttribute    *admin.UserAttributeHandler
 }
 
 // Handlers contains all HTTP handlers