Escape rendered suggestion content and validate URLs

Endpoints/EndpointHelpers.cs

@@ -164,6 +164,13 @@ internal static class EndpointHelpers
         return data[8..].StartsWith(brandBytes);
     }
 
+    public static bool IsValidHttpUrl(string? url)
+    {
+        if (string.IsNullOrWhiteSpace(url)) return true; // empty is allowed
+        if (!Uri.TryCreate(url, UriKind.Absolute, out var uri)) return false;
+        return uri.Scheme is "http" or "https";
+    }
+
     public static async Task<bool> IsAdmin(HttpContext ctx, AppDbContext db, IConfiguration config)
     {
         var player = await GetAuthenticatedPlayer(ctx, db);