Get Billing Balance

Org wallet balance snapshot for the inbox banner + composer gate.

balance_credits is the raw wallet figure (1 credit = $0.01); the derived USD value is convenient for UI rendering. The two booleans encode the thresholds the UI cares about — kept on the server so the threshold definition stays a single source of truth:

• is_empty: balance below the per-email cost. The composer pre-send precheck will return 402 in this state. The inbox banner renders red ("wallet empty — replies cannot send").
• is_low: balance below a buffer threshold (~16 emails). The inbox banner renders yellow ("low balance, top up soon"). Always true when is_empty is true.

Excludes postpaid headroom — that lives on :class:PostpaidStateResponse since it's a credit limit, not money in the user's prepaid wallet.