Add /tarifa-electrica/doble-horario endpoint

[emiliano-go]

Summary
Add a new GET /tarifa-electrica/doble-horario/ endpoint following the same pattern used by /dolar/.

Route family
This is a simple /dolar-style route.

Guide
Follow CONTRIBUTING.md, especially:

• Creating A New Indicator Route
• Simple Vs Complex Routes
• Simple /dolar-Style Routes

Reference implementation

• /dolar

Acceptance criteria

• GET /tarifa-electrica/doble-horario/ exists
• the route is visible in /docs
• the route uses the same router/service split as /dolar
• async helper calls are properly awaited
• the route returns a small JSON-friendly dict