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"`