fix(bot): skip join-link reminders without links

Prevent offline sessions with empty join links from entering the 5-minute join-link notification flow, omit blank link lines from direct reminders, and add offline persistence/reminder regression coverage.

tests/GmRelay.Bot.Tests/Infrastructure/Telegram/TelegramPlatformMessengerTests.cs

@@ -2,6 +2,7 @@ using GmRelay.Bot.Infrastructure.Telegram;
 using GmRelay.Shared.Platform;
 using GmRelay.Shared.Rendering;
 using Microsoft.Extensions.Logging.Abstractions;
+using System.Reflection;
 
 namespace GmRelay.Bot.Tests.Infrastructure.Telegram;
 
@@ -36,9 +37,35 @@ public sealed class TelegramPlatformMessengerTests
         Assert.Contains("Existing schedule message reference must match the schedule group.", exception.Message);
     }
 
+    [Fact]
+    public void BuildDirectNotificationText_OneHourReminderWithoutJoinLink_ShouldNotRenderBlankLinkLine()
+    {
+        var notification = new PlatformDirectSessionNotification(
+            PlatformDirectSessionNotificationKind.OneHourReminder,
+            new PlatformUser(PlatformKind.Telegram, "123", "Player", "player"),
+            Guid.NewGuid(),
+            "Offline Game",
+            DateTime.UtcNow,
+            JoinLink: string.Empty);
+
+        var text = InvokeBuildDirectNotificationText(notification);
+
+        Assert.DoesNotContain("🔗", text);
+    }
+
     private static TelegramPlatformMessenger CreateMessenger() =>
         new(null!, NullLogger<TelegramPlatformMessenger>.Instance);
 
+    private static string InvokeBuildDirectNotificationText(PlatformDirectSessionNotification notification)
+    {
+        var method = typeof(TelegramPlatformMessenger).GetMethod(
+            "BuildDirectNotificationText",
+            BindingFlags.NonPublic | BindingFlags.Static);
+
+        Assert.NotNull(method);
+        return Assert.IsType<string>(method.Invoke(null, new object[] { notification }));
+    }
+
     private static SessionBatchViewModel CreateView() =>
         new("Test batch", []);
 }