Reveal the decrypted withheld content of a review-hold

Return the decrypted original held message of a Safe House review-hold so an authorized human can read it before ruling (Wave 4). POST (not GET) so the SameSite=Lax session cookie makes it CSRF-safe like /resolve; the request carries no body. Customer/human auth (an authenticated member of the org that governs the review's agent, or platform staff). A review_id outside the caller's authority returns 404, never 403 (no existence leak, mirroring resolve). The decrypt happens gateway-side (keys never leave the gateway) and is strictly read-only — it never changes the hold's state. The plaintext is disclosed only after its reveal is durably audit-logged. Works for a hold in any status. Fails closed (502) on a gateway/vault error or an audit-write failure — never a fabricated or unaudited plaintext.