Add /inflacion endpoint

[emiliano-go]

Summary
Add a new GET /inflacion/ endpoint.

Route family
This is a more complex boleto-style route, not a simple /dolar-style route.

Warning for new contributors
Before working on this issue, please read CONTRIBUTING.md and strongly consider completing a simpler /dolar-like endpoint first, such as /supergas or one of the /tarifa-electrica endpoints.

Guide
Follow CONTRIBUTING.md, especially:

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

Reference implementations

• Simple reference: /dolar
• Complex reference: /boleto

Expected pattern

• add a service in app/services/
• add a router in app/routers/
• export the router from app/routers/init.py
• register the router in app/main.py
• keep parsing logic in the service layer
• return structured JSON that is easy to consume

Acceptance criteria

• GET /inflacion/ exists
• the route is visible in /docs
• the route follows the /boleto-style grouped parsing pattern when needed
• async work is properly awaited
• selectors and parsing assumptions are clear in code
• the response is structured JSON, not raw HTML