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/repository/wire.go

@@ -36,6 +36,8 @@ var ProviderSet = wire.NewSet(
 	NewUsageLogRepository,
 	NewSettingRepository,
 	NewUserSubscriptionRepository,
+	NewUserAttributeDefinitionRepository,
+	NewUserAttributeValueRepository,
 
 	// Cache implementations
 	NewGatewayCache,