fix: close web access to foreign groups and sessions

src/GmRelay.Web/Services/ClaimsPrincipalExtensions.cs

@@ -0,0 +1,9 @@
+using System.Security.Claims;
+
+namespace GmRelay.Web.Services;
+
+public static class ClaimsPrincipalExtensions
+{
+    public static bool TryGetTelegramId(this ClaimsPrincipal user, out long telegramId) =>
+        long.TryParse(user.FindFirst("TelegramId")?.Value, out telegramId);
+}