Create an Errors project and receive its ingest address once

Creates a project under the same plan allowance the dashboard enforces and mints its ingest key. The 201 response carries dsn (the native ingest address) and sentryDsn (the same key as a Sentry-shaped DSN) and is the only time either is returned: the plaintext key is not stored, so a later GET /errors/projects shows only keyHint. Both addresses are safe to put in a browser bundle; they can only submit events to this one project. Project names are labels, not identifiers, so list the projects before retrying a request whose response was lost.