Harden owner and suggestion invariants for concurrent writes

Endpoints/EndpointHelpers.cs

@@ -1,5 +1,6 @@
 using GameList.Data;
 using GameList.Domain;
+using Microsoft.Data.Sqlite;
 using Microsoft.EntityFrameworkCore;
 using System.Net;
 using System.Net.Sockets;
@@ -9,6 +10,9 @@ namespace GameList.Endpoints;
 
 internal static class EndpointHelpers
 {
+    public const string SingleOwnerIndexName = "IX_Players_IsOwner";
+    public const string SuggestionLimitTriggerError = "suggestion_limit_exceeded";
+
     public static async Task<Player?> GetAuthenticatedPlayer(HttpContext ctx, AppDbContext db)
     {
         if (ctx.User.Identity?.IsAuthenticated != true)
@@ -108,6 +112,20 @@ internal static class EndpointHelpers
 
     public static IResult UnauthorizedError(string detail = "Unauthorized") => Problem(StatusCodes.Status401Unauthorized, "Unauthorized", detail);
 
+    public static bool IsSqliteConstraintViolation(DbUpdateException ex)
+    {
+        return ex.InnerException is SqliteException sqliteEx
+            && sqliteEx.SqliteErrorCode == 19;
+    }
+
+    public static bool IsSqliteConstraintViolation(DbUpdateException ex, string containsMessage)
+    {
+        if (!IsSqliteConstraintViolation(ex))
+            return false;
+
+        return ex.InnerException?.Message.Contains(containsMessage, StringComparison.OrdinalIgnoreCase) == true;
+    }
+
     private static IResult Problem(int statusCode, string title, string detail)
     {
         return Results.Problem(