repo(card): SetProtected with zero-count-row preservation

Implements ICardInventoryRepository.SetProtected — loads only the
owned-cards collection (no decks/currency), guards against the EF
owned-nav Card.Id==0 default-init quirk, and accepts Count=0 rows
(destruct→re-protect round-trip). Covered by 4 new NUnit tests
(flip, unset, zero-count-row, unknown-card error). Full suite: 533/533.

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

SVSim.Database/Repositories/Card/CardInventoryRepository.cs

@@ -154,4 +154,21 @@ public class CardInventoryRepository : ICardInventoryRepository
 
         return CreateOutcome.Ok(new CreateResult(viewer.Currency.RedEther, allGrants));
     }
+
+    public async Task<ProtectOutcome> SetProtected(long viewerId, long cardId, bool isProtected)
+    {
+        // Lighter load than create/destruct: only need viewer's owned-cards collection. No decks,
+        // no currency, no CollectionInfo.
+        var viewer = await _db.Viewers
+            .Include(v => v.Cards).ThenInclude(c => c.Card)
+            .FirstAsync(v => v.Id == viewerId);
+
+        var owned = viewer.Cards.FirstOrDefault(c => c.Card.Id == cardId);
+        if (owned is null || owned.Card.Id == 0)
+            return ProtectOutcome.Fail(ProtectError.UnknownCard);
+
+        owned.IsProtected = isProtected;
+        await _db.SaveChangesAsync();
+        return ProtectOutcome.Ok();
+    }
 }