Support RFC 7591 - Dynamic Client Registration

# Feature

This RFC itself is quite short, implementing the ([client registration endpoint](https://tools.ietf.org/html/rfc7591#section-3) will do the trick.

## Context

OAuth2 is a framework that works great but needs clients to handle a `client_id` and sometimes `client_secret`. While this is not a problem with centralized services, this easily becomes a limitation for self-hosted solutions. Trying to register a Wallabag app against a Wallabag self-hosted instance can give you a good idea of how difficult it is for end-users.

Fortunately, the [RFC 7591 - Dynamic Client Registration](https://tools.ietf.org/html/rfc7591) allows clients to ask the server for a `client_id` and `client_secret`, making it easy to use for end-users of self-hosted apps.

## Alternatives

As a former UK PM said, _there is no alternative_

The issue references RFC 7591 and implementing a client registration endpoint. Start by reading the RFC to understand the protocol. Look at the existing OAuth2 server code in the repository, likely under src/endpoint or similar, to see how endpoints are structured. Check for existing registration or client management code. 'Done' means a new endpoint that accepts registration requests and returns client credentials, integrated with the server's frontend (like actix).

Escrita pelo modelo de indexação a partir do texto da issue.