Subscriptions: one shared app and API allowance with optional top-ups

Provide a simple free allowance and one monthly subscription, with the same included usage available through the app and authenticated API. Keep the existing account ledger and optional prepaid top-ups.

Product decisions

• One account balance and allowance; no separate developer subscription or API bill.
• Account shows balance and direct Top up / Transfer / Usage links. Usage includes transaction history.
• Pricing clearly states what is free, the monthly price, included allowance and what happens at the cap.
• $40/month is a candidate from discussion, not an agreed or cost-validated price.
• Prefer payment on our site where practical; Stripe-hosted checkout is acceptable. Minimise manual Stripe catalogue setup.
• Keep developer details out of the ordinary subscription flow. No pay-per-article model in this scope.

Remaining work

• Measure actual model/tool costs and realistic light/heavy usage; choose sustainable free and paid allowances, concurrency and spending limits.
• Verify Stripe's supported recurring-payment and embedded-checkout options; automate any required product/price setup rather than assuming these objects can be avoided.
• Define allowance consumption order, reset/renewal, expiry/rollover, cap behaviour and optional top-ups. No automatic paid overage by default.
• Implement subscription lifecycle in account/: start, renew, payment failure, cancellation and expiry, with signed/idempotent webhooks and restart-safe grants.
• Apply the same entitlement checks to app and authenticated API use; preserve existing prepaid balances and service metering.
• Keep x402 pay-per-call settlement distinct; do not grant subscription access based on an unrelated payer address.
• Update Account and Pricing together with concise labels and an explicit monthly commitment.
• Verify duplicate/out-of-order webhook delivery, cancellation, renewal, allowance exhaustion, top-up and existing-account migration in Stripe test mode.

Depends on the delivery/reliability work in #1804 and #1823. v2.1.0 still uses daily quota and prepaid credits; this issue does not claim subscriptions are implemented.

Start by reading the account/ subscription area and the existing Account and Pricing flows, then review dependencies #1804 and #1823. Use Stripe test mode to verify lifecycle, webhook, allowance, top-up, migration, and duplicate-delivery behavior; the work is done when the listed subscription decisions are implemented for both app and authenticated API use.