Renewal flow (per-subscription):
- New /subscriptions/:id/renew page showing only period options for
the specific tariff with balance display and renew button
- PurchaseCTAButton links to /renew in multi-tariff mode
Purchase flow (new tariffs only):
- Hide already purchased tariffs (is_purchased flag from backend)
- Fallback UI when all tariffs are owned
- Types: added is_purchased, all_tariffs_purchased fields
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