## Because
- `/subscriptions` and `/subscriptions/products/:productId` still run in Backbone, so they block the move of post-verify routes to React.
- The earlier plan kept these routes in Backbone until FXA-6117. This PR moves them now instead, behind the staged rollout flag.
## This pull request
- Adds a `SubscriptionsRedirect` page in fxa-settings for `/subscriptions` and `/subscriptions/products/:productId`.
- Keeps parity with Backbone: `allowUnauthenticatedRedirects` sends signed-out users to checkout, `?signin` forces sign-in first, and `usePaymentsNextSubscriptionManagement` picks `subscriptions/landing`.
- Serves both routes from React through `postVerifyOtherRoutes` in the content-server react-app routes, with `fullProdRollout: false`.
- Passes the `subscriptions` config from content-server to fxa-settings.
- Makes a bare `?signin` force sign-in (`pageParams.has('signin')`), the same as the Backbone parser. One small difference: an empty `?signin=` now also forces sign-in, and Backbone did not. That fails safe.
## Issue that this pull request solves
Closes: FXA-6661