Return the current credit balance and usage statistics for inbox-placement testing.

Credits are split into two distinct pools:

• Monthly pool: Recurring credits granted from active subscription plans (monthly_balance / monthly_allotment). Resets to allotment each billing month with no rollover.
• Pay-as-you-go pool: Credits purchased outright as packs or migrated (payg_balance). Never expires.

When running inbox placement tests, credits drain from monthly pools first (ordered by soonest expiry) before consuming non-expiring pay-as-you-go credits.

Note — usage vs. balances. used_this_month reflects total debits across both pools during the current UTC calendar month, whereas monthly_balance and monthly_allotment track the active billing cycle, which resets at next_reset_at. Do not combine these figures for balance arithmetic mid-cycle.