postResponseForTenant warmer endpoint trust

Trap: authorizing cache warming from protocol alone looks safe, but Anthropic-compatible sessions may use foreign origins—MiniMax reports anthropic while using api.minimax.io—and cached bodies intentionally retain no admin credentials. Fix: bind warming to the trusted normalized upstream origin and clear the cached body when extraHeadersForUpstream() finds no matching trusted headers; never fall back to api.anthropic.com for a foreign host.