Issue #3619275: Build the processed endpoint from the configured API prefix

The API prefix became a setting, and one place kept the literal: the inbound path processor decided whether a request was the endpoint it exists for by comparing the path against "/api/v1/get-path-by-alias". On a site that moved the API that endpoint was routed and never processed - the alias was not resolved onto the endpoint serving the entity, the redirect the front end reads was not set, and the rules answering 404 for the prefix of the default language never ran. Nothing errored; the endpoint simply answered the wrong thing.

The constant now holds the name of the endpoint and the path is built from the prefix, through a new endpoint() on the path matcher, so that nothing has to join a prefix and a name and get the slashes right on its own.

Asking the same question of the rest of the module found two more: the paragraph structures handed the front end an endpoint of their own as a literal string, and the redirect subscriber still carried a prefix constant, dead since it started asking the matcher. Both are gone.

Closes #3619275