Redeem a partner code on the packages step.

<br/>A trial code grants the capped trial entitlement, exactly as the MCP<br/>paywall route already did. A plan_grant code creates a real Stripe<br/>subscription discounted to zero, so a design partner ends up with the full<br/>package and every downstream surface treats them as a normal subscriber.<br/><br/>Rate limited on the same backoff bucket as the MCP redeem route, because<br/>this endpoint is the other door into the same guessable secret.