Finish connecting a Drupal site with the emailed code

Signed with the site key: `signature` is the Ed25519 signature, in base64url, over the key-sorted canonical JSON of the body without `signature`. For a reachable site, Decionis fetches `https://<site>/.well-known/decionis-challenge` over HTTPS with no redirects, a five-second budget and a 16 KB cap. The site answers a JSON object with `protocol` (`decionis-drupal-connect/1`), `purpose` (`challenge`), `connect_id`, `site_url`, `challenge`, and `signature`: the site key's signature over the rest, signed the same way. A match claims the challenge, once, and connects the site (200). A site that answers without a valid proof is refused (403 DRUPAL_CHALLENGE_FAILED) and is not offered a code. For a site Decionis cannot reach, or that gives no answer at all, a 6-digit code is emailed to the billing address instead (202); finish with `connect/code`. Until a code sender is configured, that answers 503 DRUPAL_EMAIL_CODE_UNAVAILABLE. Connecting binds the site to one Decionis organization for good, creating it the first time, and answers that organization's least-privilege key (role MEMBER, scopes decision:read and decision:write) exactly once: only its hash is kept. Proving a connected site again issues a new key and revokes the one it had. The site starts in Shadow. The challenge is single-use. A second verify of a used challenge answers 409 DRUPAL_CHALLENGE_ALREADY_USED, and a connect that has connected, failed or sent a code answers 409 DRUPAL_CONNECT_NOT_PENDING: start a new one. When Decionis refuses to bind the site, the answer carries that status with UPSTREAM_UNAVAILABLE.