feat(billing): add prepaid payments and receipts (Phase 5 Payments)

[law-chain-hot]

Summary

• add payment setup, manual top-up, auto-reload, and receipt records
• provide Fake payments for Dev and a Stripe provider for external test-mode validation
• protect top-ups with idempotency keys and verified provider results
• keep ambiguous provider responses pending for safe retry
• reject public webhooks when the Fake provider is active

Stack

• Depends on feat(billing): expose usage and wallet views (Phase 4 Billing UI) #974
• This is the final Billing V3 layer; enforcement remains out of scope

Validation

• Billing API: 63/63 passed
• PostgreSQL edge cases: 7/7 passed
• Dashboard: 39/39 passed
• scoped lint and API/Dashboard builds passed
• local Fake-provider browser flow passed

External validation pending

• Real Stripe test-mode Checkout and webhook flow have not been run

Review

Review only the diff against codex/billing-v3-pr4-billing-ui.

[coderabbitai]

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/billing-v3-pr5-payment

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