feat(inventory): add ViewerAcquireHistoryEntry entity + DbSet

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>

SVSim.Database/Models/ViewerAcquireHistoryEntry.cs

@@ -0,0 +1,30 @@
+namespace SVSim.Database.Models;
+
+/// <summary>
+/// One row per grant emitted by <c>InventoryTransaction.CommitAsync</c>. Rendered as the
+/// <c>histories[]</c> array on <c>POST /item_acquire_history/info</c>. Capped at 300 rows
+/// per viewer; oldest pruned on commit.
+/// </summary>
+public sealed class ViewerAcquireHistoryEntry
+{
+    public long Id { get; set; }
+    public long ViewerId { get; set; }
+
+    /// <summary>UserGoodsType cast to int; matches the wire <c>reward_type</c>.</summary>
+    public int RewardType { get; set; }
+
+    /// <summary>Detail id for the goods; 0 for wallet currencies.</summary>
+    public long RewardDetailId { get; set; }
+
+    /// <summary>Delta granted in this row — NOT a post-state total.</summary>
+    public int RewardCount { get; set; }
+
+    /// <summary>GrantSource cast to int; matches the wire <c>acquire_type</c>.</summary>
+    public int AcquireType { get; set; }
+
+    /// <summary>Pre-localized text the client renders verbatim. Capped at 64 chars.</summary>
+    public string Message { get; set; } = string.Empty;
+
+    /// <summary>Server UTC at commit time. Stamped once per <c>CommitAsync</c>, identical across all rows in that commit.</summary>
+    public DateTime AcquireTime { get; set; }
+}

SVSim.Database/SVSimDbContext.cs

@@ -103,6 +103,7 @@ public class SVSimDbContext : DbContext
 
     public DbSet<ViewerPresent> ViewerPresents => Set<ViewerPresent>();
     public DbSet<TutorialPresentEntry> TutorialPresentEntries => Set<TutorialPresentEntry>();
+    public DbSet<ViewerAcquireHistoryEntry> ViewerAcquireHistory => Set<ViewerAcquireHistoryEntry>();
 
     public DbSet<ArenaTwoPickReward> ArenaTwoPickRewards { get; set; } = null!;
     public DbSet<ViewerArenaTwoPickRun> ViewerArenaTwoPickRuns { get; set; } = null!;
@@ -398,6 +399,19 @@ public class SVSimDbContext : DbContext
             b.Property(p => p.PresentId).HasMaxLength(64);
         });
 
+        modelBuilder.Entity<ViewerAcquireHistoryEntry>(b =>
+        {
+            b.HasKey(e => e.Id);
+            b.Property(e => e.Id).ValueGeneratedOnAdd();
+            b.Property(e => e.Message).HasMaxLength(64).IsRequired();
+            b.HasOne<Viewer>()
+                .WithMany()
+                .HasForeignKey(e => e.ViewerId)
+                .OnDelete(DeleteBehavior.Cascade);
+            b.HasIndex(e => new { e.ViewerId, e.AcquireTime, e.Id })
+                .HasDatabaseName("IX_ViewerAcquireHistory_ViewerId_AcquireTime_Id");
+        });
+
         base.OnModelCreating(modelBuilder);
     }