Payment integration

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Problem: No mechanism exists to actually collect payment from a customer and tie it to their compute-time allocation.

Area: Product / Billing

Proposal: Integrate a payment mechanism (fiat and/or on-network, per #4781's economics decision) that feeds directly into #4790's pricing conversion.

Deliverables:

• A working payment flow resulting in a correctly allocated compute-time balance.

Acceptance criteria:

• A test payment results in the correct, verifiable allocation via Stake/payment → compute-time pricing #4790.

Boundaries:

• Blocked on Stake/payment → compute-time pricing #4790. No real payment credentials or customer financial data referenced in code comments, commit messages, or issue/PR text — handle exclusively through the payment provider's own secret-storage mechanism.

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Part of #4778.

[linear-code]

JSO-3475