fix(discord): treat /newsession and /reschedule input as Moscow time (UTC+3)

DiscordNewSessionHandler.ParseTimeInput used DateTimeStyles.AssumeUniversal,
which interpreted user input as UTC. A user entering 15:00 got a session
scheduled at 18:00 MSK after rendering. Align with Telegram behavior by
treating input as Moscow time and converting to UTC before storage.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/GmRelay.Bot.Tests/Discord/DiscordNewSessionHandlerTests.cs

@@ -17,6 +17,17 @@ public sealed class DiscordNewSessionHandlerTests
 
     // --- Runtime tests for ParseTimeInput (static, no DB) ---
 
+    [Fact]
+    public void ParseTimeInput_ShouldTreatInputAsMoscowTime()
+    {
+        var result = DiscordNewSessionHandler.ParseTimeInput("2026-06-01 15:00");
+        Assert.True(result.IsSuccess);
+        // 15:00 MSK = 12:00 UTC
+        Assert.Equal(12, result.Value.Hour);
+        Assert.Equal(0, result.Value.Minute);
+        Assert.Equal(TimeSpan.Zero, result.Value.Offset);
+    }
+
     [Fact]
     public void ParseTimeInput_ShouldParseDiscordDateFormat()
     {
@@ -28,7 +39,8 @@ public sealed class DiscordNewSessionHandlerTests
         Assert.Equal(expected.Year, result.Value.Year);
         Assert.Equal(expected.Month, result.Value.Month);
         Assert.Equal(expected.Day, result.Value.Day);
-        Assert.Equal(19, result.Value.Hour);
+        // Input is treated as Moscow time; 19:30 MSK = 16:30 UTC
+        Assert.Equal(16, result.Value.Hour);
         Assert.Equal(30, result.Value.Minute);
     }