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).

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"