Billing status endpoint

GET /openai/v1/balance returns billing status for the authenticated key. For customer-organization keys, the organization balance is the only money account. Project budgets are reporting targets and are never returned as spendable balance.

A valid key returns 200 OK:

{
  "object": "balance",
  "currency": "USD",
  "billing_subject": "organization_shared_balance",
  "mode": "organization_balance",
  "balance": 54.75,
  "remaining": 54.75,
  "organization_balance": 60,
  "reserved": 5.25,
  "unlimited": false
}

For organization-funded keys, organization_balance is the book balance, reserved is the amount held for admitted requests that have not settled, and balance / remaining are the currently available amount. A missing or unreconciled organization wallet returns 503 balance_unavailable; it is never reported as a fabricated zero.

An optional key_spend_limit describes a key-level restriction when one is explicitly configured. It does not create or reserve money. Subscription, contract-invoice, and external billing contexts keep their corresponding public modes.