Adds the ViewerAcquireHistoryEntry model (8 fields: Id, ViewerId, RewardType, RewardDetailId, RewardCount, AcquireType, Message, AcquireTime), registers DbSet<ViewerAcquireHistoryEntry> on SVSimDbContext, configures model (PK, FK cascade to Viewer, MaxLength 64 on Message, composite index on ViewerId/AcquireTime/Id), and adds a DbSet round-trip integration test. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>