List marketing audiences (scope: marketing:read)

Add a newsletter subscriber from a public form, with double opt-in: the address stays pending and receives ONE confirmation email; nothing else is sent until the person confirms. Idempotent per address (201 created, 200 already known; a confirmed address is never re-mailed and a pending one at most every 10 minutes). Rate limit 600/h per organisation. Honours Idempotency-Key. (scope: marketing_subscribers:write) POST