As a dev, I want to deduplicate character limits between the frontend and backend

[Samuel-Therrien-Beslogic]

As a follow-up to #337 (review)

We want to deduplicate the character limit to have a single source of truth.

I think the best approach is to have a "backend configuration" or "backend information" endpoint that we can poke once at the application startup.
Such an endpoint could eventually be used to deduplicate SiteType enum (frontend) / Sitetype model (backend) as well.

I'm open to other suggestions.