Change signature from (Viewer, packId, cardId) to (IInventoryTransaction, packId, cardId).
Drop RewardGrantService from GachaPointService ctor. PackController.ExchangeGachaPoint opens
tx with GachaPointBalances/Received extra includes, passes tx, commits on success.
Update GachaPointServiceTests to use inv.BeginAsync + tx pattern.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>