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/domain/auth_models.go

@@ -80,6 +80,7 @@ type UserProfileResponse struct {
 	AffiliationType        string         `json:"affiliationType"`
 	CompanyCode            string         `json:"companyCode,omitempty"`
 	TenantID               *string        `json:"tenantId,omitempty"`       // 추가
+	SessionTenantID        *string        `json:"sessionTenantId,omitempty"` // [New] 로그인에 사용된 식별자 기반 테넌트
 	RelyingPartyID         *string        `json:"relyingPartyId,omitempty"` // 추가
 	Metadata               map[string]any `json:"metadata,omitempty"`
 	Tenant                 *Tenant        `json:"tenant,omitempty"`

backend/internal/domain/idp_models.go

@@ -12,11 +12,12 @@ var ErrNotSupported = errors.New("idp: not supported")
 // BrokerUser is the standard user model used within Baron SSO business logic.
 // It defines the canonical set of fields that must be supported by any underlying IDP.
 type BrokerUser struct {
-	ID          string `json:"id" required:"true"`
-	Email       string `json:"email" required:"true"`
-	LoginID     string `json:"login_id"`
-	Name        string `json:"name"`
-	PhoneNumber string `json:"phone_number"`
+	ID             string   `json:"id" required:"true"`
+	Email          string   `json:"email" required:"true"`
+	LoginID        string   `json:"login_id"`
+	CustomLoginIDs []string `json:"custom_login_ids"` // [New] 다중 로그인 ID
+	Name           string   `json:"name"`
+	PhoneNumber    string   `json:"phone_number"`
 	// Attributes stores custom user attributes.
 	// The "required_keys" tag specifies which keys MUST be present in the IDP's schema support.
 	Attributes map[string]interface{} `json:"attributes" required_keys:"grade,department"`

backend/internal/domain/user.go

@@ -35,14 +35,13 @@ func NormalizeRole(role string) string {
 type User struct {
 	ID              string         `gorm:"primaryKey;type:uuid;default:gen_random_uuid()" json:"id"`
 	Email           string         `gorm:"uniqueIndex;not null" json:"email"`
-	LoginID         string         `gorm:"column:login_id;uniqueIndex:idx_tenant_login_id" json:"loginId"`
 	PasswordHash    *string        `gorm:"column:password_hash" json:"-"`
 	Name            string         `gorm:"column:name;not null" json:"name"`
 	Phone           string         `gorm:"column:phone" json:"phone"`
 	Role            string         `gorm:"column:role;default:'user';not null" json:"role"` // super_admin, tenant_admin, rp_admin, user
 	AffiliationType string         `gorm:"column:affiliation_type" json:"affiliationType"`
 	CompanyCode     string         `gorm:"column:company_code;index" json:"companyCode"`
-	TenantID        *string        `gorm:"column:tenant_id;type:uuid;index;uniqueIndex:idx_tenant_login_id" json:"tenantId,omitempty"`
+	TenantID        *string        `gorm:"column:tenant_id;type:uuid;index" json:"tenantId,omitempty"`
 	Tenant          *Tenant        `gorm:"foreignKey:TenantID" json:"tenant,omitempty"`
 	RelyingPartyID  *string        `gorm:"column:relying_party_id;type:uuid;index" json:"relyingPartyId,omitempty"` // RP Admin용
 	Department      string         `gorm:"column:department" json:"department"`
@@ -53,6 +52,18 @@ type User struct {
 	CreatedAt       time.Time      `gorm:"column:created_at" json:"createdAt"`
 	UpdatedAt       time.Time      `gorm:"column:updated_at" json:"updatedAt"`
 	DeletedAt       gorm.DeletedAt `gorm:"column:deleted_at;index" json:"-"`
+
+	// Multiple identifiers support
+	UserLoginIDs []UserLoginID `gorm:"foreignKey:UserID" json:"userLoginIds,omitempty"`
+}
+
+// UserLoginID represents multiple custom identifiers for a user
+type UserLoginID struct {
+	ID       string `gorm:"primaryKey;type:uuid;default:gen_random_uuid()" json:"id"`
+	UserID   string `gorm:"type:uuid;not null;index" json:"userId"`
+	TenantID string `gorm:"type:uuid;not null;index" json:"tenantId"` // 발급 테넌트
+	FieldKey string `gorm:"not null" json:"fieldKey"`                 // 스키마 필드 키 (예: emp_id)
+	LoginID  string `gorm:"uniqueIndex;not null" json:"loginId"`      // 실제 값 (예: EMP001)
 }
 
 // BeforeCreate hook to generate UUID if not present