feat: add node export API for Markdown and HTML formats

[qianeric-backup]

Summary

Add node export API that allows users to export knowledge base nodes in Markdown or HTML format with automatic format conversion.

Changes

• Add NodeExportReq struct with id, kb_id, and format parameters
• Implement ExportNode usecase method with format conversion support
• Add convertHTMLToMarkdown helper for HTML to Markdown conversion
• Add ExportNode handler with proper validation and error handling
• Register GET /api/v1/node/export endpoint

API Usage

GET /api/v1/node/export?id={node_id}&kb_id={kb_id}&format=md
GET /api/v1/node/export?id={node_id}&kb_id={kb_id}&format=html

Test Plan

• Test Markdown format export
• Test HTML format export
• Test automatic format conversion
• Test invalid parameter handling