Stripe webhook for mu top-up grants (signature-authenticated upstream)

Stripe → api → mnemom-mu grant path (MNE-2516). Accepts Stripe webhook deliveries (e.g. payment_intent.succeeded) and forwards them BYTE-VERBATIM to the ledger's /v1/stripe/webhook consumer, which verifies the stripe-signature header against its webhook secret and fail-closes (401 bad signature, 503 unconfigured). api adds no authentication of its own — the payload, not the transport, is what is authenticated — and no ledger logic. Purchase grants land exactly-once with Stripe provenance; retail grant creation happens ONLY through this path.