feat(inventory): prune acquire history above 300-row cap

Adds PruneAcquireHistoryAsync to InventoryTransaction.CommitAsync; runs
inside the open DB transaction after history rows are flushed, keeping at
most 300 rows per viewer (oldest discarded). Adds a covering test that
verifies the cap and per-viewer isolation.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

SVSim.Database/Services/Inventory/InventoryTransaction.cs

@@ -9,6 +9,8 @@ namespace SVSim.Database.Services.Inventory;
 
 internal sealed class InventoryTransaction : IInventoryTransaction
 {
+    private const int AcquireHistoryRetention = 300;
+
     private readonly SVSimDbContext _db;
     private readonly IDbContextTransaction _dbTx;
     private readonly ILogger _log;
@@ -282,6 +284,8 @@ internal sealed class InventoryTransaction : IInventoryTransaction
         WriteAcquireHistory();
         await _db.SaveChangesAsync(ct);
 
+        await PruneAcquireHistoryAsync(ct);
+
         await _dbTx.CommitAsync(ct);
         _committed = true;
 
@@ -290,6 +294,22 @@ internal sealed class InventoryTransaction : IInventoryTransaction
         return new InventoryCommitResult(rewardList, deltas);
     }
 
+    private async Task PruneAcquireHistoryAsync(CancellationToken ct)
+    {
+        var overflowIds = await _db.ViewerAcquireHistory
+            .Where(h => h.ViewerId == Viewer.Id)
+            .OrderByDescending(h => h.AcquireTime).ThenByDescending(h => h.Id)
+            .Skip(AcquireHistoryRetention)
+            .Select(h => h.Id)
+            .ToListAsync(ct);
+
+        if (overflowIds.Count == 0) return;
+
+        await _db.ViewerAcquireHistory
+            .Where(h => overflowIds.Contains(h.Id))
+            .ExecuteDeleteAsync(ct);
+    }
+
     private void WriteAcquireHistory()
     {
         var now = DateTime.UtcNow;