Fix document links and backlinks rendering issue

- Fixed CachedAPI endpoint routing: /documents/{id}/links was incorrectly routed to getDocument()
- Added proper pattern matching for document API calls using regex
- Fixed link and backlink API calls to use correct getDocumentLinks() and getDocumentBacklinks() methods
- Added comprehensive debugging logs for API response tracking
- Resolved issue where links were not rendering in document viewer

Changes:
- cached-api.js: Fixed endpoint routing with proper regex pattern matching
- link-manager.js: Enhanced debugging and API response handling
- viewer-core.js: Added closeAllModals() to prevent modal auto-opening

Result: Document links and backlinks now render correctly (8 links restored)

frontend/static/js/viewer/utils/cached-api.js

@@ -53,8 +53,20 @@ class CachedAPI {
             } else if (endpoint === '/document-links/backlinks' && params.target_document_id) {
                 // 실제: /documents/{documentId}/backlinks
                 response = await this.api.get(`/documents/${params.target_document_id}/backlinks`);
-            } else if (endpoint.startsWith('/documents/') && endpoint.includes('/')) {
+            } else if (endpoint.startsWith('/documents/') && endpoint.endsWith('/links')) {
+                // /documents/{documentId}/links 패턴
                 const documentId = endpoint.split('/')[2];
+                console.log('🔗 CachedAPI: 링크 API 직접 호출:', documentId);
+                response = await this.api.getDocumentLinks(documentId);
+            } else if (endpoint.startsWith('/documents/') && endpoint.endsWith('/backlinks')) {
+                // /documents/{documentId}/backlinks 패턴
+                const documentId = endpoint.split('/')[2];
+                console.log('🔗 CachedAPI: 백링크 API 직접 호출:', documentId);
+                response = await this.api.getDocumentBacklinks(documentId);
+            } else if (endpoint.startsWith('/documents/') && endpoint.match(/^\/documents\/[^\/]+$/)) {
+                // /documents/{documentId} 패턴만 (추가 경로 없음)
+                const documentId = endpoint.split('/')[2];
+                console.log('📄 CachedAPI: 문서 API 호출:', documentId);
                 response = await this.api.getDocument(documentId);
             } else {
                 // 기본 API 호출 (기존 방식)