Buy compute credits up front, with auto-reload

You can now buy compute credits up front rather than settling overage after the fact, and set auto-reload so a long-running agent doesn't stall mid-task. Auto-reload takes a monthly cap, so topping up automatically can't run away from you.

Memory API usage now reconciles into the same credit balance as MemHub — one number for both products instead of two meters to reason about. Credit purchases appear on your invoice, and the top-up history is filterable by date range.