Deliver an inbound webhook

Public receipt endpoint that accepts a signed payload from an external service. The platform verifies the signature using the connection manifest’s webhook_verification.method adapter, deduplicates against the sender’s external_delivery_id (a unique index on (subscription_id, external_delivery_id) collapses repeat presentations to the original receipt row), and enqueues the envelope for the per-integration Worker. Failed verification returns 401 invalid_signature and the body is not forwarded. See Inbound webhooks.