refactor: allow production license URL override

[Aditya-Rai-25]

Issue No: #6

This PR refactors the production license verification flow to support a configurable license verification URL.

The LAGO_LICENSE_URL environment variable can now be used to override the default production Lago license verification URL. This allows Mango Cloud deployments to route license verification requests to a custom entitlement API server, which can validate the configured LAGO_LICENSE key.

Changes include:

add support for LAGO_LICENSE_URL environment variable
use the custom license URL when LAGO_LICENSE_URL is configured
keep the existing production license verification URL as the default fallback
allow custom entitlement API integration for license validation

This change is required for Mango Cloud billing integration, where license verification needs to be handled by a custom entitlement API server instead of the default upstream Lago license verification endpoint.

[njonty]

The PR removes the existing staging/Lago Cloud behavior:

LAGO_CLOUD=true + RAILS_ENV=staging previously used http://license-web.default.svc.cluster.local

Now it always uses LAGO_LICENSE_URL or falls back to https://license.getlago.com . That may break staging cloud deployments unless they explicitly set LAGO_LICENSE_URL.

Suggested fix:
config.license_url = ENV.fetch("LAGO_LICENSE_URL") do
if ENV["LAGO_CLOUD"] == "true" && ENV["RAILS_ENV"] == "staging"
"http://license-web.default.svc.cluster.local"
else
"https://license.getlago.com"
end
end