Prepaid-credit balance for the calling API key

Returns the authenticated API key's OWN prepaid-credit balance in whole øre — the same per-key balance the 402 INSUFFICIENT_CREDITS meter debits. This is the Bearer/agent-facing member of the /account family (every other /api/v1/account/* endpoint is session-cookie-authed for the browser dashboard): the balance ledger is keyed per API key, so the key itself is the resource owner, and the endpoint takes NO parameters — the only balance it can ever return is the calling key's own, making cross-key reads structurally impossible. Read-only with zero side effects: no accrual, no consume, no ledger row. top_up_url is byte-identical to the top_up_url the 402 INSUFFICIENT_CREDITS error body advertises (one server-side builder), so an agent can hard-code the handover from either surface. updated_at is the timestamp of the last balance movement and is null when the key has never been topped up (no balance row yet — the balance is authoritatively 0, not unknown). Deliberately exempt from the scope model: this is the caller's own billing metadata, not a data-source read, and a key that just received 402 must be able to check its balance regardless of how narrowly its scopes are cut.