feat(server): answer the root path and probe container health

A bare GET / previously returned an empty 404, which reads as an outage
even while the service is healthy. The root now redirects to
/diagnostics when the dashboard is enabled and otherwise returns a
small JSON status pointing at /health/live. The route is excluded from
the OpenAPI description, so the frozen v1 surface is unchanged.

The chiseled runtime image ships no shell or curl, so the container
healthcheck re-enters the server assembly with --health-probe, which
queries the local liveness endpoint and reports through the exit code.
Dockerfile and compose both wire the probe (30s interval, 3 retries).

Dockerfile

@@ -32,4 +32,8 @@ COPY --from=build --chown=1654:1654 /out/ ./
 USER 1654:1654
 EXPOSE 8080/tcp
 EXPOSE 9050/udp
+# The chiseled image has no shell or curl; the probe re-enters the server
+# assembly in --health-probe mode and reports through the exit code.
+HEALTHCHECK --interval=30s --timeout=5s --start-period=15s --retries=3 \
+    CMD ["dotnet", "FinalFactory.Rendezvous.Server.dll", "--health-probe"]
 ENTRYPOINT ["dotnet", "FinalFactory.Rendezvous.Server.dll"]

deploy/compose/compose.yaml

@@ -24,6 +24,12 @@ services:
         hard: 4096
     stop_grace_period: 40s
     restart: unless-stopped
+    healthcheck:
+      test: ["CMD", "dotnet", "FinalFactory.Rendezvous.Server.dll", "--health-probe"]
+      interval: 30s
+      timeout: 5s
+      retries: 3
+      start_period: 15s
     environment:
       ASPNETCORE_ENVIRONMENT: Production
       ASPNETCORE_HTTP_PORTS: "8080"

src/FinalFactory.Rendezvous.Server/FinalFactory.Rendezvous.Server.csproj

@@ -57,11 +57,13 @@
     <Compile Include="Diagnostics/DiagnosticDashboardOptions.cs" />
     <Compile Include="Http/ContractEndpoints.cs" />
     <Compile Include="Http/RendezvousExceptionHandler.cs" />
+    <Compile Include="Http/RootEndpoints.cs" />
     <Compile Include="JoinAttempts/JoinAttemptCursorCodec.cs" />
     <Compile Include="JoinAttempts/JoinAttemptService.cs" />
     <Compile Include="Observability/AuditOptions.cs" />
     <Compile Include="Observability/AuditTrail.cs" />
     <Compile Include="Observability/HealthEndpoints.cs" />
+    <Compile Include="Observability/HealthProbe.cs" />
     <Compile Include="Observability/PrometheusMetricsEndpoint.cs" />
     <Compile Include="Observability/RendezvousReadiness.cs" />
     <Compile Include="Observability/RendezvousTelemetry.cs" />

src/FinalFactory.Rendezvous.Server/Http/RootEndpoints.cs

@@ -0,0 +1,27 @@
+using FinalFactory.Rendezvous.Contracts;
+using FinalFactory.Rendezvous.Server.Diagnostics;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.Extensions.Options;
+
+namespace FinalFactory.Rendezvous.Server.Http;
+
+internal static class RootEndpoints
+{
+    public static IEndpointRouteBuilder MapRootEndpoint(
+        this IEndpointRouteBuilder endpoints)
+    {
+        endpoints.MapGet("/", ServeRoot).ExcludeFromDescription();
+        return endpoints;
+    }
+
+    private static IResult ServeRoot(
+        [FromServices] IOptions<DiagnosticDashboardOptions> diagnostics) =>
+        diagnostics.Value.Enabled
+            ? Results.Redirect("/diagnostics")
+            : Results.Json(new
+            {
+                contractVersion = ContractLimits.ContractVersion,
+                service = "rendezvous",
+                health = "/health/live",
+            });
+}

src/FinalFactory.Rendezvous.Server/Observability/HealthProbe.cs

@@ -0,0 +1,34 @@
+namespace FinalFactory.Rendezvous.Server.Observability;
+
+/// <summary>
+/// Container healthcheck entry mode (<c>--health-probe</c>). The chiseled
+/// runtime image ships no shell or curl, so the probe re-enters the server
+/// assembly, queries the local liveness endpoint, and reports via exit code.
+/// </summary>
+internal static class HealthProbe
+{
+    public const string Argument = "--health-probe";
+
+    public static async Task<int> RunAsync()
+    {
+        string configuredPorts =
+            Environment.GetEnvironmentVariable("ASPNETCORE_HTTP_PORTS") ?? "8080";
+        string port = configuredPorts
+            .Split(';', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries)
+            .FirstOrDefault() ?? "8080";
+        using HttpClient client = new() { Timeout = TimeSpan.FromSeconds(4) };
+        try
+        {
+            using HttpResponseMessage response = await client.GetAsync(
+                new Uri($"http://127.0.0.1:{port}/health/live"));
+            return response.IsSuccessStatusCode ? 0 : 1;
+        }
+        catch (Exception exception)
+            when (exception is HttpRequestException
+                or TaskCanceledException
+                or UriFormatException)
+        {
+            return 1;
+        }
+    }
+}

src/FinalFactory.Rendezvous.Server/Program.cs

@@ -16,6 +16,11 @@ using FinalFactory.Rendezvous.Server.Transport;
 using Microsoft.AspNetCore.HttpOverrides;
 using Microsoft.OpenApi;
 
+if (args.Contains(HealthProbe.Argument, StringComparer.Ordinal))
+{
+    return await HealthProbe.RunAsync();
+}
+
 WebApplicationBuilder builder = WebApplication.CreateBuilder(args);
 builder.Logging.AddFilter(
     "Microsoft.AspNetCore.Diagnostics.ExceptionHandlerMiddleware",
@@ -378,6 +383,7 @@ app.UseMiddleware<TelemetryMiddleware>();
 app.UseExceptionHandler();
 app.UseMiddleware<HttpAbuseProtectionMiddleware>();
 app.MapOpenApi();
+app.MapRootEndpoint();
 app.MapRendezvousContractEndpoints();
 app.MapOperatorEndpoints();
 app.MapRendezvousHealthEndpoints();
@@ -385,6 +391,7 @@ app.MapDiagnosticDashboardEndpoints();
 app.MapPrometheusMetricsEndpoint(metricsOptions, metricsCredential);
 
 await app.RunAsync();
+return 0;
 
 /// <summary>
 /// Entry point marker used by integration-test hosts.