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/web/src/components/knowledge/index.ts

@@ -6,3 +6,4 @@ export { EntryViewer } from "./EntryViewer";
 export { EntryEditor } from "./EntryEditor";
 export { EntryMetadata } from "./EntryMetadata";
 export { VersionHistory } from "./VersionHistory";
+export { ImportExportActions } from "./ImportExportActions";