feat: add health check endpoints for Bot and Web

- Web: add /health endpoint with PostgreSQL readiness check (returns 200+JSON or 503)
- Web: add /alive endpoint for liveness probe
- Bot: add BotHealthCheckHostedService serving /health on port 8081 via HttpListener
- Bot: expose port 8081 in Dockerfile and install wget for healthcheck
- compose.yaml: add healthcheck sections for bot and web services
- tests: add TDD tests for both health endpoints

Bump version -> 1.16.0

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

src/GmRelay.Web/Health/NpgsqlHealthCheck.cs

@@ -0,0 +1,25 @@
+using Microsoft.Extensions.Diagnostics.HealthChecks;
+using Npgsql;
+
+namespace GmRelay.Web.Health;
+
+public sealed class NpgsqlHealthCheck(NpgsqlDataSource dataSource) : IHealthCheck
+{
+    public async Task<HealthCheckResult> CheckHealthAsync(
+        HealthCheckContext context,
+        CancellationToken cancellationToken = default)
+    {
+        try
+        {
+            await using var connection = await dataSource.OpenConnectionAsync(cancellationToken);
+            await using var command = connection.CreateCommand();
+            command.CommandText = "SELECT 1";
+            await command.ExecuteScalarAsync(cancellationToken);
+            return HealthCheckResult.Healthy();
+        }
+        catch (Exception ex)
+        {
+            return HealthCheckResult.Unhealthy("PostgreSQL is unavailable", ex);
+        }
+    }
+}