Redeem Promo Code

Redeem a system-generated referral code.

Validates the code against the referral_codes table (case-insensitive), then grants RevenueCat premium for the duration configured on the code (week / month / year), stacking on top of any existing subscription. The grant is recorded in referral_grants.

Restrictions:

• Code must exist in referral_codes and have status = 'active'
• User cannot redeem the same code more than once (no limit on redeeming different codes)

Args: request: RedeemSystemCodeRequest with the code

Returns: RedeemSystemCodeResponse with success status and message