Account billing snapshot (plan, cycle, balance, caps, status)

The billing "wallet/statement" view: current plan, billing cycle, accrued balance + remaining credits this period, spend caps, and payment / access status. This is the billing half of the legacy /v1/usage-stats snapshot — the per-product consumption half is metering and lives on GET /v1/usage.

Usage-based (Metronome) accounts get a populated balance; legacy Stripe accounts get balance: null plus a deprecated legacy.limits block and, when payment-blocked, status.openInvoiceUrl / status.declineReason.