Sandbox test API keys and interactive Playground launched

Every project now gets a mm_test_ API key alongside its live mm_live_ key, for integrating without spending real credits or sending real messages. Test-mode OTPs use a fixed code (000000), skip WhatsApp/SMS delivery entirely, and carry a higher rate-limit budget suited to development traffic. The dashboard’s new Playground lets you try the full send → verify flow interactively, without writing any code first.