Reveal manual connection credentials

Delivers the OAuth client secret for a website's manual connection - the only response on this surface that reveals it. Never build this URL by hand: always follow `meta.reveal_url` exactly as returned by "Manual connection credentials", which mints it as a short-lived Laravel signed URL (`signed:relative`) carrying `user`, `nonce`, `expires` and `signature`. This route opts out of personal-access-token authentication entirely (`AuthenticatePublicApi` is skipped): the signed URL itself is the credential, so `Authorization` is ignored here. The link is single-use: reading it once spends the `nonce`, independently of its time-to-live. Route name `public.v1.sites.connection.manual.reveal`. Answers **200** with `meta.client_id` and `meta.client_secret` - the plaintext OAuth client secret used to finish the manual WordPress connection. Errors: **403** with a JSON:API `errors` document when the signature is invalid, tampered or expired (Laravel's own "Invalid signature." message), and the same status when the nonce was already spent ("This link has already been used or has expired.", the message the API returns when the link is followed a second time).