fix: settle usage beyond available balance

[hrx114514x]

Summary

• persist billable usage even when the final charge exceeds the available balance
• allow usage and period-overage settlement to produce a negative balance
• preserve reservation reconciliation and block later paid requests through the existing balance check

Fixes #471.

Root cause

The final settlement transactions returned ErrInsufficientBalance when the actual charge exceeded the remaining balance after reservation. That rolled back both the usage ledger and the balance update. The HTTP layer then treated settlement as failed and released the reservation, restoring the account to a usable balance even though the upstream request had already consumed tokens.

Changes

• treat completed upstream usage as authoritative and settle the full amount in the existing account-row transaction
• record negative balances instead of rolling back usage and balance transactions
• apply the same behavior to usage billing and period-plan overage settlement
• retain normal refunds when actual usage is lower than the reservation
• add coverage for settlement without a reservation, settlement beyond a reservation, reservation refunds, free models, period overage debt, and rejection of later calls from a negative-balance account

No API, schema, configuration, or frontend changes are included.

Concurrency and behavior

The existing account row lock remains the serialization point for balance updates and period-credit recalculation. Requests that are already in flight can still complete and are charged in commit order, potentially increasing the negative balance; once a settlement records debt, subsequent paid-model access is rejected by the existing balance check.

This pull request intentionally does not add per-model maximum-cost estimation or change the prepaid amount configuration.

Testing

• cd backend && go test ./...
• cd backend && go test -race ./internal/infra/persistence/postgres/billing ./internal/application/billing
• git diff --check