feat: implement multi-identifier architecture (Issue #496)

- Database: Add user_login_ids table for 1:N identifier mapping and remove legacy login_id column
- Kratos: Update identity schema to use custom_login_ids array instead of a single id trait
- Backend: Implement syncCustomLoginIDs to collect isLoginId fields across tenant schemas
- Backend: Add backtracking logic to auto-assign session tenant based on used login identifier
- Backend: Add 409 Conflict exception handling for Create/Update operations
- AdminFront: Refactor UserDetailPage to a tabbed grid layout (Info, Tenants, Security)
- AdminFront: Show '로그인 ID' badge on tenant schema fields used for authentication
- UserFront: Remove legacy optional 'Login ID' input from signup flow
- Tests: Add multi-identifier repository tests and update handler tests

backend/internal/bootstrap/bootstrap.go

@@ -34,6 +34,7 @@ func migrateSchemas(db *gorm.DB) error {
 		&domain.Tenant{},
 		&domain.TenantDomain{},
 		&domain.User{},
+		&domain.UserLoginID{},
 		&domain.UserGroup{},
 		&domain.ApiKey{},
 		&domain.IdentityProviderConfig{},