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/Program.cs

@@ -5,6 +5,7 @@ using KArtSell.BuildingBlocks.Capabilities;
 using Microsoft.Extensions.Caching.Memory;
 using KArtSell.Host.Jobs;
 using KArtSell.Host.Configuration;
+using KArtSell.Host.Infrastructure;
 using KArtSell.BuildingBlocks.Data;
 using KArtSell.BuildingBlocks.Reliability;
 using KArtSell.BuildingBlocks.Time;
@@ -18,14 +19,28 @@ using OpenTelemetry.Metrics;
 using OpenTelemetry.Resources;
 using OpenTelemetry.Trace;
 using Serilog;
+using Serilog.Events;
 
 var builder = WebApplication.CreateBuilder(args);
 
-builder.Host.UseSerilog((context, services, logger) => logger
-    .ReadFrom.Configuration(context.Configuration)
-    .ReadFrom.Services(services)
-    .Enrich.FromLogContext()
-    .WriteTo.Console());
+// Load Telegram secrets for Serilog notifications
+var telegramBotToken = Environment.GetEnvironmentVariable("TELEGRAM_BOT") ?? string.Empty;
+var telegramChatId = Environment.GetEnvironmentVariable("CHAT_ID") ?? string.Empty;
+
+builder.Host.UseSerilog((context, services, logger) =>
+{
+    var config = logger
+        .ReadFrom.Configuration(context.Configuration)
+        .ReadFrom.Services(services)
+        .Enrich.FromLogContext()
+        .WriteTo.Console();
+
+    // Add Telegram sink for ERROR and FATAL logs
+    if (!string.IsNullOrEmpty(telegramBotToken) && !string.IsNullOrEmpty(telegramChatId))
+    {
+        config = config.WriteTo.Sink(new TelegramSink(telegramBotToken, telegramChatId), LogEventLevel.Error);
+    }
+});
 
 // Load secrets from environment variables (set by CI/CD or user-secrets in dev)
 var connectionString = ResolveSecret(