Realtime tier: live WebSocket tick stream

New Realtime tier ($80/mo): a WebSocket stream at wss://api.goldprice.dev/v1/stream pushing live XAU and XAG spot ticks in real time, up to 10 concurrent streams per key and 1M calls/mo, with all Pro features included. The Realtime plan is closed to new sign-ups; existing subscribers keep it. Connections are authenticated with your ga_live_ API key. REST polling and the Pro SSE oracle stream (/v1/prices/stream) are unchanged.