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/utils/audit.go

@@ -0,0 +1,17 @@
+package utils
+
+import (
+	"encoding/json"
+	"fmt"
+)
+
+func ParseAuditDetails(details string) (map[string]any, error) {
+	var payload map[string]any
+	if details == "" {
+		return nil, fmt.Errorf("empty details")
+	}
+	if err := json.Unmarshal([]byte(details), &payload); err != nil {
+		return nil, err
+	}
+	return payload, nil
+}