Serve Swagger UI at root

[rikedyp]

Serve Swagger UI as the service home page:

• Bundle static assets or use a CDN-referenced single HTML page
• Register GET / to serve it, pointing at /openapi.json
• This is the public-facing API documentation

Acceptance criteria:

•  GET / returns Swagger UI in a browser
•  All endpoints visible with schemas, request/response examples
•  Try-it-out works against the running service