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/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"`