Confirm a quarantined message as a threat

Marks the item confirmed_threat for arena sideband analysis. Does not write to sh_threat_patterns — the quarantine store holds only a content hash, and the pattern table requires plaintext. Authorization: the caller must be authenticated, resolve to an organization and hold the owner, admin or member role in it (viewers and auditors are read-only and get 403), and quarantine_id is matched scoped to that org's own agents, so a record owned by another tenant returns 404 and no other tenant's item can be reported.