Harden owner and suggestion invariants for concurrent writes

GameList.Tests/SuggestionTests.cs

@@ -1,6 +1,7 @@
 using System.Net;
 using System.Net.Http.Json;
 using System.Text.Json;
+using GameList.Domain;
 using GameList.Tests.Support;
 using Microsoft.EntityFrameworkCore;
 
@@ -626,4 +627,41 @@ public class SuggestionTests
             Assert.False(db.Votes.Any(v => v.SuggestionId == id));
         });
     }
+
+    [Fact]
+    public async Task Suggestion_limit_is_enforced_by_database_trigger_without_joker()
+    {
+        await using var factory = new TestWebApplicationFactory();
+        var client = factory.CreateClientWithCookies();
+        await client.RegisterAsync("dbcap");
+
+        var playerId = await factory.WithDbContextAsync(async db => await db.Players.Select(p => p.Id).SingleAsync());
+
+        await factory.WithDbContextAsync(async db =>
+        {
+            for (var i = 0; i < 5; i++)
+            {
+                db.Suggestions.Add(new Suggestion
+                {
+                    PlayerId = playerId,
+                    Name = $"Seed {i}"
+                });
+            }
+
+            await db.SaveChangesAsync();
+        });
+
+        var thrown = await Assert.ThrowsAsync<DbUpdateException>(() => factory.WithDbContextAsync(async db =>
+        {
+            db.Suggestions.Add(new Suggestion
+            {
+                PlayerId = playerId,
+                Name = "Blocked by trigger"
+            });
+
+            await db.SaveChangesAsync();
+        }));
+
+        Assert.Contains("suggestion_limit_exceeded", thrown.InnerException?.Message ?? thrown.Message, StringComparison.OrdinalIgnoreCase);
+    }
 }