fix: CORS duplicate header + Neo4j query + DateTime serialization

- Remove CORS headers from nginx (let Flask-CORS handle it alone)
- Fix Neo4j edge query to match all relationship types (RELATES_TO + MENTIONS)
- Convert Neo4j DateTime objects to strings for JSON serialization
- Add .serena/, .vercel/, .venv_direct/ to gitignore

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

.gitignore

@@ -59,3 +59,6 @@ backend/uploads/
 # Docker 数据
 data/backend/.venv_direct/
 .playwright-mcp/
+.serena/
+.vercel/
+backend/.venv_direct/