Add /ui endpoint

[emiliano-go]

Summary
Add a new GET /ui/ endpoint following the same pattern used by /dolar/.

Guide
Follow CONTRIBUTING.md, especially the section named Creating A New Indicator Route.

Expected pattern

• add a new service in app/services/
• add a new router in app/routers/
• export the router from app/routers/init.py
• register the router in app/main.py
• inject HttpxClientDep in the route function signature
• keep scraping logic in the service layer
• return a JSON-friendly dict from the route

Acceptance criteria

• GET /ui/ exists
• the route is visible in /docs
• the route uses the same router/service split as /dolar/
• async helper calls are properly awaited
• the endpoint returns structured JSON, not a coroutine and not raw HTML unless intentionally designed that way
• the scraping selector is documented in the code if it is not obvious

Notes for contributors
Start by copying the /dolar flow and rename everything carefully. The easiest safe path is:

1. create the service
2. create the router
3. export the router
4. register the router
5. test in Swagger UI

If the value is inside a span, prefer get_span_element(). If it is inside a div, prefer get_div_element().