feat: add markdown import/export (closes #77, #78)

- Add POST /api/knowledge/import endpoint for .md and .zip files
- Add GET /api/knowledge/export endpoint with markdown/json formats
- Import parses frontmatter (title, tags, status, visibility)
- Export includes frontmatter in markdown format
- Add ImportExportActions component with drag-and-drop UI
- Add import progress dialog with success/error summary
- Add export dropdown with format selection
- Include comprehensive test suite
- Support bulk import with detailed error reporting

apps/api/package.json

@@ -57,6 +57,8 @@
     "@nestjs/schematics": "^11.0.1",
     "@nestjs/testing": "^11.1.12",
     "@swc/core": "^1.10.18",
+    "@types/adm-zip": "^0.5.7",
+    "@types/archiver": "^7.0.0",
     "@types/express": "^5.0.1",
     "@types/highlight.js": "^10.1.0",
     "@types/ioredis": "^5.0.0",