e1f5b9b6c3
Implements GachaPointService.TryExchangeAsync: validates pack exchangeability, balance >= threshold, card in catalog, not already received; debits balance, marks received, grants the card through RewardGrantService (cascade handles cosmetics). Re-adds the RewardGrantService injection that was removed in the Task 3 fix-up (matches the "inject when you call" convention). Card grant produces the wire-shape reward_list directly via the cosmetic cascade — the catalog's reward_list remains the display-only shape for /pack/get_gacha_point_rewards. Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>