[ABB65]

Stacked on #542 (prepaid Studio year core). Review that first; this PR's base is its branch and moves to main once it merges.

What this adds

Billing page and banner

• "Prepaid through {date}" replaces the monthly price during a prepaid year.
• From 30 days before the end: "Keep Studio" on Starter or Pro yearly (owner/admin; members see "ask the owner"). Before that, the date it opens. After choosing, the scheduled plan, date and price, plus Manage Subscription.
• The trial banner says when the prepaid year ends and links to Keep Studio, only inside the window and only while no renewal is set.

Keep Studio (POST /api/billing/keep-studio, server/utils/migrate-prepaid-renewal.ts)

• Yearly checkout with a trial that ends with the prepaid year (clamp(ceil(days left), 1, 30)), so nothing is charged before the year ends.
• Metadata migrate_renewal_grant_id, never migrate_grant_id: the webhook links it to the grant (renewal_subscription_id), no redeem, no duplicate check. trial_origin: 'prepaid_renewal' is not a trial for trial context or trial reminders.
• Refusals: prepaid_not_active, renewal_not_open, renewal_scheduled. An ordinary checkout during a prepaid year now answers with the prepaid message.
• Polar: yearly product per plan. Stripe: interval: 'year' is refused (not configured there).

Year-end sweep (server/plugins/migrate-prepaid-sweep.ts, every 6 h)

• Mails at T−30, T−7 and the end; forward-only cursor migrate_grants.prepaid_reminder_stage (migration 056, CHECK 0–3).
• A year with a renewal gets no "choose a plan" mail. At the end the year comes off through endPrepaidYear, then the ended mail.
• New templates: prepaid-ending, prepaid-ended, prepaid-renewed.

Honest copy

• Overage stays locked during the year with its own reason prepaid (settings GET/PATCH, usage, usage panel).
• The limit-reached text, the usage-alert next step and the lock text say the year is prepaid instead of "change plan in Billing".
• Usage alerts read prepaid_plan / prepaid_until (F2).
• No "now on the free plan" canceled mail while the prepaid year runs; at renewal conversion the activation mail is the yearly prepaid-renewed mail.
• The usage period of a renewal trial stays the calendar month, so the free quota is not reset by the trial's billing key.

Revoke residual

• Revoke cancels a renewal with the year.
• A repeated revoke takes off a year an earlier call could not.

Attach

• In the last 30 days of a prepaid year with no renewal, attach is refused (attach_prepaid_ending).
• With a renewal, the renewal plan must cover the grant.

Claim page

• A prepaid grant says "{plan}, prepaid through {date}". No cancel note: prepaid grants are bundles.

Docs

• docs/PAYMENT_PROVIDERS.md: "Prepaid Studio year".

Known edges (not in this PR)

• Keep Studio opens its usage companion subscription at checkout, a month or less before the year ends. It bills nothing while overage is locked.
• Cancelling a renewal before the year ends sends the generic "cancel scheduled" mail wording.
• If the stage write fails after the year is taken off, the ended mail for that year is not retried.

Tests

• Unit: renewal checkout, sweep, attach, trial cap, usage period, usage alerts, email templates, checkout trial days, revoke route.
• Integration: webhook (renewal link, cancel during the year, yearly activation mail), overage settings.
• Contract: prepaid grants (renewal, cursor, window), usage alerts with a prepaid workspace.
• Nuxt: the trial banner prepaid window.