Make the API path prefix and the generated plugin IDs configurable so the module can coexist with an existing REST layer

Problem/Motivation

Coexistence is one of the guarantees this module has to make: on a site that already serves a REST API, installing this module must not collide with it. Today the path /api/v1/… is hardcoded in every resource plugin annotation, and the plugin IDs are chosen by hand. Once endpoints are generated from a content model, both become mechanical, and a collision with an existing resource ID or route stops being a theoretical risk.

Proposed resolution

• Add api_path_prefix to myrest.settings, defaulting to the current /api/v1, and build routes from it via the route subscriber rather than from the literal in each annotation.
• Give generated plugin IDs a configurable namespace so that two generations, or a generation next to a handwritten resource, cannot claim the same ID.
• Detect a collision at scan time and report it rather than at route rebuild, where the error is unreadable.
• Keep the existing paths and IDs byte for byte on the sites that already run this module — the compatibility profile pins them.

Remaining tasks

Everything. This issue is a precondition for the generator: without it, generated resources cannot be installed next to an existing API.