feat: Serilog Telegram Integration for Alert Notifications

Add automatic Telegram notifications for ERROR and FATAL level logs.

Features:
- TelegramSink: Custom Serilog sink for Telegram API integration
- Conditional logging: Only ERROR and FATAL levels trigger alerts
- Environment variables: TELEGRAM_BOT and CHAT_ID from Gitea Secrets
- Non-blocking: Telegram failures don't crash application

Configuration:
- Reads TELEGRAM_BOT and CHAT_ID from environment
- Formatted messages with emoji, timestamp, and exception details
- Markdown parsing for better Telegram presentation

This enables real-time alerting for critical issues during:
- Gate 3 Shadow Run execution
- Production deployments
- System errors and exceptions

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/KArtSell.Host/Infrastructure/TelegramSink.cs

@@ -0,0 +1,106 @@
+using System.Net.Http;
+using Serilog;
+using Serilog.Configuration;
+using Serilog.Core;
+using Serilog.Events;
+
+namespace KArtSell.Host.Infrastructure;
+
+/// <summary>
+/// Serilog sink for sending critical logs to Telegram
+/// Triggers on ERROR and FATAL events
+/// </summary>
+public sealed class TelegramSink : ILogEventSink
+{
+    private readonly string _telegramBotToken;
+    private readonly string _telegramChatId;
+    private readonly HttpClient _httpClient;
+    private readonly object _syncRoot = new();
+
+    public TelegramSink(string telegramBotToken, string telegramChatId, HttpClient? httpClient = null)
+    {
+        _telegramBotToken = telegramBotToken;
+        _telegramChatId = telegramChatId;
+        _httpClient = httpClient ?? new HttpClient();
+    }
+
+    public void Emit(LogEvent logEvent)
+    {
+        // Only send critical logs (Error and Fatal)
+        if (logEvent.Level < LogEventLevel.Error)
+            return;
+
+        try
+        {
+            lock (_syncRoot)
+            {
+                SendTelegramMessage(logEvent);
+            }
+        }
+        catch
+        {
+            // Silently ignore Telegram errors to prevent logging loops
+        }
+    }
+
+    private void SendTelegramMessage(LogEvent logEvent)
+    {
+        var emoji = logEvent.Level == LogEventLevel.Fatal ? "🔴" : "⚠️";
+        var levelName = logEvent.Level.ToString().ToUpperInvariant();
+
+        var message = $@"{emoji} *{levelName}* - K-ArtSell Aegis
+
+{logEvent.MessageTemplate.Render(logEvent.Properties)}
+
+_Timestamp: {logEvent.Timestamp:O}_";
+
+        if (logEvent.Exception != null)
+        {
+            message += $@"
+
+```
+{logEvent.Exception.GetType().Name}: {logEvent.Exception.Message}
+```";
+        }
+
+        SendMessage(message);
+    }
+
+    private void SendMessage(string message)
+    {
+        var url = $"https://api.telegram.org/bot{_telegramBotToken}/sendMessage";
+
+        var content = new FormUrlEncodedContent(new Dictionary<string, string>
+        {
+            { "chat_id", _telegramChatId },
+            { "text", message },
+            { "parse_mode", "Markdown" }
+        });
+
+        try
+        {
+            var response = _httpClient.PostAsync(url, content).GetAwaiter().GetResult();
+            response.EnsureSuccessStatusCode();
+        }
+        catch
+        {
+            // Silently fail - don't want logging to break application
+        }
+    }
+}
+
+/// <summary>
+/// Serilog extension for adding Telegram sink
+/// </summary>
+public static class TelegramSinkExtensions
+{
+    public static LoggerConfiguration Telegram(
+        this LoggerSinkConfiguration loggerConfiguration,
+        string telegramBotToken,
+        string telegramChatId,
+        HttpClient? httpClient = null)
+    {
+        return loggerConfiguration.Sink(
+            new TelegramSink(telegramBotToken, telegramChatId, httpClient));
+    }
+}