Top up Layerbeat credit with USDC

Funding and VM purchases are separate actions. USDC adds credit to your personal wallet; you explicitly submit a VM purchase once sufficient credit is available.

1. GET /v1/billing/payment-methods lists the enabled funding networks.
2. POST /v1/billing/top-ups with {"method":"usdc_solana","amount":"25.00"} creates funding instructions. No VM or operation is created.
3. Send the displayed USDC amount on the displayed network. Use payment_uri for a wallet link or QR; compatible Solana wallets include the unique payment reference.
4. Poll GET /v1/billing/payments/{id} and GET /v1/billing/wallet. Only finalized, attributed funds become spendable credit.
5. Choose the VM region, plan, image and SSH key, review a fresh quote, then submit POST /v1/vms. The request spends available credit; omit payment_method or set it to wallet.

Partial and overpayments become credit. Late receipts also become credit while the closed intent stays closed. Fractional USDC is retained across the user's receipts; only whole USD cents are available to spend under the current 1:1 accounting policy. Funding instructions expire at expires_at; expiration does not remove credited funds.

Only the configured USDC token and network are accepted. Exact-amount matching is available for reference-free transfers; ambiguous or unattributable receipts are recorded for verified support review. New instructions exclude previously issued amounts. There are 9,999 tags per USD-cent amount/network; if they are exhausted, choose a different top-up amount. References are preferable because historical duplicate amounts cannot identify their original customer reliably.

A top-up returns 503 funding_unavailable if its initial scanning baseline cannot be persisted. Retry later. A confirmed receipt never starts a VM automatically.