CVE-2026-47127: fallo de gravedad media en ghostfolio

Ghostfolio is an open source wealth management software. Prior to version 3.4.0, Ghostfolio's Stripe checkout success-URL handler at `GET /api/v1/subscription/stripe/callback?checkoutSessionId=<id>` retrieves the Stripe Checkout Session by ID and unconditionally grants a Premium subscription to the session's `client_reference_id` — without ever checking `session.payment_status` or `session.status`. There is no separate Stripe webhook endpoint with `stripe-signature` verification; this callback is the sole code path that creates Stripe-driven subscriptions. Any authenticated user can self-grant a 1-year Premium subscription without ever paying. Version 3.4.0 rejects sessions unless `session.payment_status === 'paid'` AND `session.status === 'complete'` (fails closed). Additionally, new unique `stripeCheckoutSessionId` column → a session can't be redeemed twice (race-safe via DB unique constraint).